Messages be changed from sound into electricity before they be sent by radio.
A. must; will
B. have to; can
C. can; must
D. need; may
网友回答
A. must; will
(答案→)B. have to; can
C. can; must
D. need; may
解析：本句意思是“信息得从声音转化成电子信号才能被送出去”。must和have to都可以翻译成“必须”，往往可以互用，但must表示的是说话人的主观看法，而have to则强调客观需要，如：The film is not interesting. I really must go now.
I had to work when I was your age.
本题很明显是强调客观需要。