It is necessary that a college student __________ a foreign language.

A. should master
B. be mastered
C. mastered
D. should mastering

He took his raincoat with him in case it __________.

A. rain
B. should rain
C. rains
D. had been rained

He is not a pilot, but he wishes he ____(be).

He ordered the work ____(start) at once.