The three-part inference spectrum
Must Be True (MBT), Could Be True (CBT), and Cannot Be True (CNBT) are three points on a spectrum of certainty. MBT is the strongest claim — something that is definitely true given the information. CNBT is at the other end — something that is definitely impossible given the information. CBT sits in the middle — something that is merely consistent with the information, not proven but not ruled out either.
Understanding all three as a connected spectrum is more useful than learning each one in isolation, because many of the trap answers on any one type exploit the gap between the three. A common MBT wrong answer is actually a CBT (it could be true but isn't certain). A common CNBT wrong answer is actually a CBT (it's not established, but it's not ruled out either).
Must Be True: what the correct answer must do
On a Must Be True question, the correct answer follows necessarily from the information in the stimulus. It cannot be false if the stimulus is true. This is a strict logical requirement — not 'probably true,' not 'consistent with,' but guaranteed.
The test for an MBT answer: can I construct a scenario where the stimulus information is true and this answer is false? If yes, the answer is not MBT — it might be CBT or it might be wrong entirely. Only if there is no possible scenario where the answer is false, given the premises, does it qualify as must be true.
Common MBT traps: answers that go further than the stimulus supports (the stimulus says 'most', the answer says 'all'), answers that are plausible but not provable from the given information, and answers that are true in the real world but not established by the specific premises given.
Could Be True: what the correct answer must do
On a Could Be True question, the correct answer is merely consistent with the stimulus — it's possible, not proven. The information doesn't establish it as true, but it also doesn't rule it out.
CBT questions are less common than MBT but appear regularly. Their defining feature: the wrong answers are things that cannot be true (they are ruled out or contradict the stimulus), and the correct answer is the one that doesn't contradict anything.
This makes CBT feel backward to MBT students: instead of looking for the most supported answer, you're looking for the answer that's simply not ruled out. On CBT, a completely unsupported answer that's also not contradicted can be the correct answer.
Watch for the stem phrasing: 'Which of the following could be true based on the information above?' or 'If the statements above are all true, each of the following could be true EXCEPT.' That EXCEPT version is actually asking for a CNBT answer — the one that cannot be true.
Cannot Be True: what the correct answer must do
On a Cannot Be True question, the correct answer is logically impossible given the stimulus. It directly contradicts the information — there is no scenario where both the stimulus and the correct answer are true simultaneously.
CNBT questions feel like the logical inverse of MBT questions. Where MBT asks 'what must follow,' CNBT asks 'what is ruled out.' The strategy is similar: evaluate each answer and ask whether there's any possible scenario where the answer is true alongside the stimulus. If no such scenario exists, you've found the CNBT answer.
Common CNBT traps: answers that are simply not established or not likely (but not actually impossible), and answers that are false in the real world but not logically ruled out by the specific stimulus information. The question is not 'what is probably false?' but 'what is logically incompatible?'
The edges: where MBT shades into CBT, and CBT into CNBT
The hardest questions in this family sit at the boundaries. A stimulus that says 'most students prefer method A' establishes that more than half prefer A — it must be true that at least 51% prefer A, but it could be true that 99% prefer A. Misreading the scope of 'most' will lead you to incorrectly label CBT answers as MBT.
Similarly, a stimulus that says 'no student who prefers A also prefers B' rules out any student preferring both — but it doesn't rule out students preferring neither. An answer saying 'some students prefer neither A nor B' is not CNBT; it's CBT. The CNBT answer would be 'some students prefer both A and B' — that directly contradicts the 'no' statement.
A useful check: when you're unsure whether an answer is MBT vs. CBT, try to construct a minimal scenario where the answer is false while the stimulus is true. If you can, it's not MBT. When you're unsure whether an answer is CNBT vs. CBT, try to construct a scenario where both the answer and the stimulus are true simultaneously. If you can, it's not CNBT.
Most Strongly Supported: the practical fourth variant
Most Strongly Supported (MSS) questions behave similarly to MBT but with a lower bar. MBT requires that the correct answer follow with certainty; MSS requires that the correct answer be the most strongly supported among the five options — it may not be certain, but it's better supported than anything else.
On MSS questions, comparative evaluation matters. You're picking the most defensible inference, not necessarily a certain one. This distinction matters when a stimulus has probabilistic language — 'tends to,' 'most often,' 'in the majority of cases.' On an MBT question, an inference from 'most' that doesn't hold with certainty would be wrong. On an MSS question, it might be the best available answer.
Stem language: 'Which of the following is most strongly supported by the information above?' or 'Which of the following most logically completes the passage?' are MSS. 'Which of the following must be true?' is strictly MBT.
Frequently asked questions
How do I know which type I'm on from the stem?
'Must be true,' 'must be concluded,' 'follows logically from' → MBT. 'Could be true,' 'is consistent with,' 'could also be true' → CBT. 'Cannot be true,' 'must be false,' 'is impossible if' → CNBT. 'Most strongly supported,' 'most reasonably concluded,' 'most logically completes' → MSS. These phrasings are reliable — memorize the family of words for each type and you won't need to infer the type from context.
Why are Could Be True questions confusing for students who are strong at MBT?
MBT trains you to look for the most supported answer. CBT asks for the least-contradicted answer. Students strong at MBT often pick the CBT answer that's most supported by the stimulus — but on a CBT question, that answer may actually be MBT and therefore a wrong answer (because it's 'more than could be true'). The reversal of the evaluation frame is the source of confusion.
Do CNBT questions show up often?
Less often than MBT, but they appear consistently enough across PrepTests that you should have a practiced approach. The stimulus for CNBT questions often involves quantified statements ('all,' 'none,' 'most,' 'at least') where logical contradictions can be constructed precisely. When you see heavy quantification in the stimulus, consider whether the question might be asking for the logically impossible answer.
Is there a shortcut for MBT questions?
The most reliable shortcut is to extract the logical content of the stimulus as clearly as possible before reading the choices. If the stimulus contains a conditional ('if A then B') and a trigger ('A is true'), you know 'B must be true' — find the choice that says B. If the stimulus contains a contradiction that's resolved by the passage, the resolution is usually testable. The goal is to derive the most obvious inferences from the stimulus before the answer choices bias your thinking.
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