问题补充：
These young trees must .
A. look after well
B. be well looked after
C. take good care of
D. be taken good care
答案：
A. look after well
(答案→)B. be well looked after
C. take good care of
D. be taken good care
解析：考查被动语态。被动语态的构成是am/is/are+过去分词。These young trees是物作主语，应该用被动语态；must后应该用动词原形，am/is/are的原形是be，排除A、C；D项的固定短语缺少介词of，排除D。“照顾”look after= take care of。故选B。
