Messages ______ be changed from sound into electricity before they ______ be sent by radio.
A．must; willB．have to; canC．can; mustD．need; may
B
本句意思是“信息得從聲音轉(zhuǎn)化成電子信號才能被送出去”。must和have to都可以翻譯成“必須”，往往可以互用，但must表示的是說話人的主觀看法，而have to則強調(diào)客觀需要，如：The film is not interesting. I really must go now.
I had to work when I was your age.
本題很明顯是強調(diào)客觀需要。
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