While building a tunnel through the mountain, .
A. an underground lake was discovered
B. there was an underground lake discovered
C. a lake was discovered underground
D. the workers discovered an underground lake
网友回答
A. an underground lake was discovered
B. there was an underground lake discovered
C. a lake was discovered underground
(答案→)D. the workers discovered an underground lake
解析：句意，工人们挖隧道时，发现了一个地下湖。完整的从句为，While the workers were building a tunnel through the mountain，主句和从句中主语一致时省略从句中的主语和were。