--whom should this message be sent to

Whom在这里是动词send的宾语,所以我们可以把它换成who,因为who级可以做主语也可以做宾语.则就成了：who should this message be sent to?我们可以这样回答：My father is the one who should be sent the message to.这里to不能省略,因为要接father（换句话说就是：The message should be sent to my father)所以to 不能省略.那么答案中BD都排除.再来看A和C.如果选C就成了：My father is the one to be sent to.我们可以看到这个句子中缺少send to的主语,那么排除.
A是正确答案.
My father is the one to send it to.
it 相当于the message.很明显：My father is the one to send the message to.这样就明了多了.