请从语法角度加以说明~(1)

问题描述:

请从语法角度加以说明~(1)
39.the medicine ________cool,clean and dry.
A.must keep B.must be kept C.must be carried D.must be in
49.________ his stamps he has collected are from Canada .
A.two hundreds B.hundred of C.two hundred of D.two hundred
53.water_______ for ________ out fires.
A.use…put B.is used…put C.use…putting D.is used…putting

B 保持凉爽的意思,keep sth cool ,现在sth在前面,肯定要用被动式来表达
D 表示明确数字的时候,hundred是不用加s的,而hundreds of才是固定短语,所以不用C,用D
D 水被用于灭火,前面显然是被动式,后面因为跟在介词for后面,所以得用现在分词形式