The president has called Ebola an international crisis and a threat to national security. Why is this disease considered a threat to national security? a.) The disease is killing many people in Africa.
b.) The disease appears and disappears throughout history.
c.) The disease is killing medical workers.
d.) The disease travels across international borders when people travel who have it.

d.) The disease travels across international borders when people travel who have it.

answer; genetics;

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."