选择C.这道题目询问的是被动语态动作的执行者,相当于：
Who planted these trees last year?
但是这个题目是有问题的,既然是疑问句,问句的句型要出来,疑问词相当于火车头,后面的问句句型相当于火车车厢,我们常说物以类聚,也就是所谓的Birds of a feather flock together.火车头的后面绝对不会拖公共汽车或卡车的车厢,一定是拖火车的车厢,否则就会出轨或拖不动.所以这个句子要把were提到these trees的前面去,写成：
By whom were these trees planted last year?
除非这个句子是从属于另一个句子的名词性从句,我们才可以把疑问词放句子的中间,引导从句,后面用陈述句的语序,也就是：
I don't know by whom these trees were planted last year.
希望我能帮助你解疑释惑.