秦修筑长城是为了抵御[ ]
A.犬戎
B.匈奴
C.蒙古
D.越族
答案:B
Would you please ______ the light ? I can’t see much clearly . It is getting dark .
A.switch off
B.switch on
C.turn off
D.take on
天线放大器如果距离振子太远,则会使电缆损耗电平增大和( )。
A.产生回馈
B.增益低
C.信噪比变坏
D.抗干扰能力弱