架设天线的铁塔必须接地,并装避雷针,其作用为何?() ["防止雷击、保护铁塔","防止雷击、保护天线","防止雷击、保护发射机及操作人员","以上皆是"]
在正常情况下,一般人耳所能听到之频率范围约()。 ["1KHz~30KHz","20Hz~20KHz","50Hz~20MHz","以上皆非"]
关系式UA.B.=-E+IR满足图A.-3所示电路的下列()图。 ["图A.-3(A.)","图A.-3(B.)","图A.-3(C.)","图A.-3(D.)。"]
假台畜
鸭瘟多发生在() ["A.雏鸭","B.青年鸭","C.老龄鸭","D.成年鸭"]
阐述家畜的采精方法。