原句應(yīng)該為：
I think (that)... trees were planted last spring.
I think (that)they planted.trees last spring.
就名詞前的數(shù)量提問用how many；
do you think已經(jīng)用了疑問句的形式,所以其后的從句不用再寫成疑問語序,只要把疑問詞提前即可,若想用主動形式答案B應(yīng)改為they planted.
A,樹是在過去被種,所以用過去被動式.
如果用B的話,應(yīng)該改為：how many trees do you think they planted last spring.