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.
在这段文章里,a boy was knocked down off his bicycle by a delivery van这句被动句强调出读到文章的人最关心的事故的受害者.The boy was sent to St.Maria Hospital这句话则说明了孩子被送到医院的事实,至于是由谁（某个过路人?或肇事司机?）送的不重要.he was treated for shock and a broken arm这句被动句无须说出treat这个动作的发出者,因为在医院,伤病员自然由医务人员处理,无须罗嗦.这样,这段文章就重点突出,条理清楚了.
④ 有些动词可以有两个宾语,在用于被动结构时,可以把主动结构中的一个宾语变为主语,另一个宾语仍然保留在谓语后面.多是把间接宾语变为主语.这样句子自然些.直接宾语变为主语时,间接宾语要变为某个介词的宾语,介词to可以省略.如His father left him this house.改为This house was left (to) him by his father.
⑤ 有些动词虽为及物,但宾语并非是动作承受者,不能转换,这些动词有have,hold（容纳）,suit,fit,lack,become（适合）contain,cost,last,mean,suffice（足够）等.
⑥ 当直接宾语为反身代词、相互代词或宾语前有指代主语的物主代词时不用被动,如I shook my head.我摇摇头.
⑦ 当宾语为同源宾语（与主句指同一人）,动名词,动词不定式或一个从句时不用被动.如John enjoyed seeing the fil,.
⑧ 在一些固定说法中,有些名词和动词结合的固定说法,不能改We Chinese always keep our word.
⑨ 某些从不及物动词转化来的及物动词,直接宾语在表示动作的方式或效果时,这些动词在意思上起状语的作用,没有被动The girl kissed her boyfriend good night=The girl said good night to her boyfriend by kissing him.(这个女孩说了晚安并且亲了他的男朋友）
⑩ 表地点\处所\组织\长度\大小\数量\程度\抽象名词的词做宾语时不用被动.
⑪ 某些“不及物动词+介词”短语,walk into,listen to,sleep in,agree with,shake hands with,belong to,take part in,keep up with不能用被动.
⑫ 某些词用主动表被动：sell,miss,build,grow,look,smell,taste,sound,feel等
