- 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 which 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]).
- The answer There are two broad theories concerning what triggers a human's inevitable decline to death. is correct.
- 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") 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.