1) What happens to infrastructure when there is rapid growth? * A. Helps build it.
B. Helps improve it.
C. Puts a strain on it.
D. Reduces demand of it.

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."

i believe your answer is mountains.