21. A. Before B.
Since C.
Although D.
While
[解析]while“表示“当……的时候,在……期间”,后接进行时态。其他均不符合题意。
[例2] (2003全国卷)
I played a racquetball game against my cousin Ed last week.
It was one of the most surprising and tiring games I've ever had. When Ed first
phoned and we play, I laughed quietly, figuring on
an easy victory. ……
A.
declared B. mentioned C.
persuaded D.suggested
[解析]分析句子结构可看出we play应为宾语从句,只有suggest后的宾语从句的谓语用should加动词原形,should省略。
[例3](2004天津卷)I was leaving 49 several girls camp up to me.
A.
while B. when
C. as D. since
[解析] “be doing …when…”为常用句子结构,意思为“正在这个时候….”。所以选B.( when)。