Messages ______ be changed from sound in
问题:
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
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