There are usually aspects of passages that you will be able to diagram. As for strengthen with sufficient questions in particular, you are looking for the assumption which allows the argument to be properly drawn.
To do this, it is less important that statements are diagrammed, but essential that you understand the argument and examine each of the answer choices. The answer choice that, if negated, does not allow the argument to be properly drawn is the correct answer for these types of questions.
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