提一个英语语法点~~need与must有关

这里应该是被动语态吧，跟主动表被动没关系：

情态动词+be+及物动词的过去分词
Young trees must be watered often.
Your mistakes should be corrected right now.
The door may be locked inside.
Your homework can be handed in tomorrow.

所以应该选择B，不过你好像打错了，keep的过去分词应该是kept

主动表被动总结一下有如下情况：
① 有些动词在主动结构中，后面接不带to的不定式，但如果改为被动，则需把省略的to加上，这类动词有hear, watch, make, help, let等，如：The boss made my grandfather work 10hours a day.改成My grandfather was made to work for 10 hours a day.
② 含有宾语从句的主动结构变为被动，通常用it作为被动结构的先行主语，从句放在句子后面/也可采用另一种形式，这类动词有：know, say, believe, find, think, report等
③ 是所有的主动句都可以变换成被动句，更不是所有的被动句都可以自由变换成主动句。虽然语法原则上允许主动和被动句的互相转换，但有的句子转换后会变成不通顺或不地道的英语句子。因此，在某些题目里，这也成为判断应该用主动还是用被动的依据。
例：At 5:05 p.m. on Saturday 19th July , there was an accident at the junction of the Main Street and Panda Road when a boy was knocked down off his bicycle by a delivery van. The boy was sent to St. Maria Hospital where he was treated for shock and a broken arm.
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