Review: Topic Sentences, Answer 2.2

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Question:

The first is the wear-and-tear hypothesis that suggests the body eventually succumbs to the environmental insults of life. The second is the notion that we have an internal clock that is genetically programmed to run down. Supporters of the wear-and-tear theory maintain that the very practice of breathing causes us to age because inhaled oxygen produces toxic by-products. Advocates of the internal clock theory believe that individual cells are told to stop dividing and thus eventually to die by, for example, hormones produced by the brain or by their own genes. (from Debra Blank, "The Eternal Quest" [edited])

Answer:

The answer some scientists believe humans have an internal time clock that determines lifespan is not correct.

Explanation:

This paragraph is a straightforward description of two possibilities, neither of which is preferred over the other. In this case, it would be wrong to mention only one of the possibilities (the internal time clock or the wear-and-tear theory) in the topic sentence, or to treat it as a philosophical discussion of death itself (we all must die). As for the biology professor, he or she might very well have given an interesting lecture, but that has nothing to do with the content of the paragraph.