1. we were watching TV when the telephone_______
A. rings B. is ringing
C. was ringing D. rang

2. we were all enjoying the party _____suddenly the lights went out.
A. that B. where C. when D. how

瞬间性动词：表示不能延续的动作，即动作发生后立即结束。如：begin,arrive,borrow,buy,break,close,come,fall,dic,finish,go,leave,lend,mary,reach,receive,open,sell,start,stop,ring等。
大部分情况下瞬间动词没有进行时
有些瞬间性动词可以表示延续性词义，这时它们就可以与段时间状语连用了。例如：
I'm going away for a few days. 我要出去几天。(go away 表示travel)
I went to Paris for two weeks. 我去了巴黎两个星期。(go to表示 visit)
We go to the seaside for a week every August. 我们每年八月都要到海滨度过一个星期。(go 表示travel)
She left me waiting in the rain for an hour. 她让我在雨里等了一个小时。(leave表示cause sb. / sth. to be or remain in a particular state or position，即"使某人或某物处于某一特定的状态"。)

所以第1题要用一般过去时,第2题可知为时间状语从句,可知选C