Hi @ginaelliott, thanks for your post. Because the argument structure in this stimulus does not depend on transitive inferences drawn from sufficient and necessary claims, diagramming might not actually be the most efficient approach. Rather, let's just take a very close and careful look at the stimulus, first, and then the question stem. That should be enough to get us to the right answer.
This stimulus presents an argument, the conclusion of which is "any virus could easily become dangerous to humans." What premises are offered in support of this claim? (1) viruses can have beneficial effects (example: some kill more-complex microorganisms, some of which are deadly to humans) (2) but viruses have simple structures (3) such that replacing just a few of a beneficial virus's several million atoms could make it deadly to humans, and (4) alterations of greater complexity than this (that is, alterations of greater complexity than replacing just a few atoms) are commonly produced by random mutations.
The question stem presents a CANNOT BE TRUE question. Four of the answer choices could be true (are not inconsistent with the stimulus). The one correct answer choice CANNOT BE TRUE (is inconsistent with the information provided in the stimulus).
Answer choice (A) could be true. We just don't know anything about deadly viruses becoming beneficial to humans. There is no textual support in the stimulus to contradict this answer choice. On a CANNOT BE TRUE question like this, we have to eliminate this answer choice.
Similarly, answer choice (B) could be true. The stimulus does not tell us any information about "organisms of greater complexity than viruses," right? Thus, an answer choice about such organisms "could be true" - it is not contradicted by the stimulus / information given in the text here. Eliminate (B).
Answer choice (C) is logically identical to answer choice (B); both are about "organisms that are more complex than viruses." We just don't know anything about such organisms, based on the stimulus. So (C), too, can be eliminated.
Answer choice (D) is tricky, but it also could be true. Notice that the stimulus does not say that the ONLY way a virus can be beneficial to humans is by killing more-complex microorganisms that can be deadly to people. Thus, (D) could be true - it could be true that a virus that does not kill other viruses that are deadly to humans might nevertheless be beneficial to humans in some other way. This is not contradicted by the text in the stimulus, so it could be true. We just don't know. This answer can be eliminated.
Answer choice (E) is correct. Why? We are told that viruses sometimes kill "more-complex microorganisms," and that by doing so these viruses are beneficial to humans. This information (taken from the stimulus) is directly contradicted by answer choice (E), which says that "no virus that is deadly to organisms of greater complexity than itself is beneficial to humans." That is not what the stimulus says. This answer choice CANNOT BE TRUE given the text of the stimulus. It is therefore the correct answer on this question.
Hope this helps. Please let us know if you have any additional questions.
