Question

____these trees were planted last year ?A who B whom C by whom D by who

Answer 1

正确答案是C，此句的肯定句是：these trees were planted by somebody last year. 此处用somebody替代whom的位置，whom是宾格，句子主语是trees，树是被栽的，所以当对somebody提问时，用whom表示

Answer 2

选C by whom

把这句子弄成陈述句就容易找到答案了

陈述句：

These trees were planted () last year.

这样就知道括号里应填by sb. 这些树是被谁种的

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