问题补充：
—How often do I need to feed the fish?
－They food every day, or they will be hungry.
A. must give
B. mustn’t give
C. mustn’t be give.
D. must be given
答案：
A. must give
B. mustn’t give
C. mustn’t be give.
(答案→)D. must be given
解析：句意：你多久喂一次鱼？他们每天都必须被喂食，否则他们会饿。主语they指鱼，是动作的承受者，故用被动结构：be + done,含有情态动词的被动结构：情态动词+ be + done，结合句意及句子结构，故选D
