A：had not been heard of
B：has not been heard of
C：had not heard of
D：has not heard of
People will three days a week in the future．
“x与3的差的2倍”用代数式表示为（ ）
收音机调台选钮调到30000kHz位置时，可收到某广播电台的节目，你知道该广播电台的频率是多少吗？它发射的电磁波的波长又是多少？
