PROBLEM REGARDING QUESTION STEM

This is an interesting question! I think that by 'must be true' you're referring to 'Draw a Conclusion' or 'Inference' problems, right? These are problems where you need to pick something that's true based on the information in the argument. You do have to be careful to tell the difference between these questions, and Strengthen questions. But here are some clues.

Strengthen question stems will say that the right answer supports or strengthens the argument. Draw a Conclusion question stems will say that the right answer is supported by the argument. That is, in Strengthen questions, the logic should go like this:

right answer choice is true -> conclusion of the argument is probably true

but in Draw a Conclusion problems, the logic goes like this:

everything in the argument is true -> right answer choice must be true

In a way, they're opposites.

Also, a Strengthen argument will always have a conclusion already. That's what you're trying to strengthen. A Draw a Conclusion argument might not already have its own conclusion.