-how many trees do you think____ last spring

原句应该为：
I think (that)... trees were planted last spring.
I think (that)they planted....trees last spring.

就名词前的数量提问用how many；
do you think已经用了疑问句的形式，所以其后的从句不用再写成疑问语序，只要把疑问词提前即可，若想用主动形式答案B应改为they planted。

A,树是在过去被种，所以用过去被动式。
如果用B的话，应该改为：how many trees do you think they planted last spring.

A,树是在过去被种，所以用过去被动式。
如果用B的话，应该改为：how many trees do you think they planted last spring.

原句应该为：
I think (that)... trees were planted last spring.
I think (that)they planted....trees last spring.

就名词前的数量提问用how many；
do you think已经用了疑问句的形式，所以其后的从句不用再写成疑问语序，只要把疑问词提前即可，若想用主动形式答案B应改为they planted。

首先 要分清楚这个句子的主语 是trees 是被种 属被动句式 要用被动语态 其次 do you think 只属于一个状语 可以前置 也可以后置 所以不必考虑它的位置。原句也可以改成 Do you think how many trees were planted last spring ?

本例句中，主语是trees树，所以用被动语态。选A

主语如果是人，比如students，就可以用B

首先这是一个从句，那么要用陈述句的顺序、从这点就可以判断B是错的，用了A是被动语态，去年春天有多少树被载？