While building a tunnel through the mountain, ________
A.an underground lake was discovered　B.there was an underground lake discoveredC.a lake was discovered underground　D.the workers discovered an underground lake在线课程D
试题分析：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。
考点：本题重点考查从句中主语和谓语的省略。
点评：根据语言情景，在不引起歧义的情况下，可以将主语和谓语一并省略，从而使语言更加简洁、明快。