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：从句省略=While the workers were building…，主从句中主语一致时省略从句中的主语和were．