A simple way of looking at the difference between 'Must be true' and 'Strengthen' questions.
Correct answers to 'Must be true'
questions follow from the given argument whereas the answers to 'Strengthen'
questions lead to the argument i.e. you need the correct answer for the 'Strengthen' question to prove your argument, whereas for the 'Must be true' question, an answer can be correct only if it follows from the given argument.
Let's take a simple example to understand the difference. Consider the argument given below.
All my neighbors own blue cars. Therefore I own a blue car.
The argument given above is fairly weak and could be attacked using points such as 'the given statement that all my neighbors have blue car has no effect on the car I own'. Hence in order to strengthen
this particular argument we can use a statement such as 'None of the cars in my neighborhood is other than blue' which will support the fact that if I own a car, it must be blue. Note that this statement fills a logical gap in the given argument and hence strengthens it.
Now, a statement such as 'None of my neighbors own a black car' will follow from the argument. This is something that can be derived from the given facts/premises and hence it could be a correct answer to a 'Must be true'
Hope this helps you get a basic understanding of the difference. Powerscore has detailed explanation of each type of question - so you might want to go back and try to understand it carefully.
MBA Candidate 2015 | Georgetown University
McDonough School of Business