Color TV was first shown in 1929. It took more than two decades, ________， until 1951, for

color broadcasts to begin in the USA.

A. though       B. therefore      C. otherwise      D. anyway

下列不属于电子战的内容的是（）。

A、电子侦察与反侦察

B、电子干扰与反干扰

C、电子辐射与反辐射

D、电子摧毁与反摧毁