Question

【题目】Thousands of trees must _______ every year.
A.plant
B.be planted
C.have been planted
D.have planted

Answer 1

【答案】B
【解析】句意：成千上万的树每年必须被种植。must是情态动词，其后必须跟动词原形。A种植，主动形式；B被种植，被动形式；C已经被种植，与every year矛盾；D已经种植，与every year矛盾。所以正确答案是B。