Contrast the case made for the savannah theory of bipedalism (also known as the "radiator theory") with the case Elaine Morgan makes for an aquatic role in human evolution. Which do you think is most plausible, and why?

a is the final ! : )

d. i assume the underlined word is "role." reformat if it isn't.

explanation:

role is closest to part because when replacing words, "playing a vital decision," "playing a vital activity," and "playing a vital dollar" don't make logical sense. the only meaningful choice is "playing a vital part."

this fits one of merriam-webster's definitions of role: "a function or part performed especially in a particular operation or process."

answer; reincarnation of an eternal soul;