Building such a bridge over the bay was ______ but the local government made it within two years.
A. a wet blanket B. a piece of cake C. a dark horse D. a hard nut to crack
D
【解析】
【详解】考查习惯用语辨析。句意:在海湾上建造这样一座桥是一件困难的事情，但当地政府在两年内完成了。A. a wet blanket一件扫兴的事情；B. a piece of cake小菜一碟；C. a dark horse出人意外的获胜者；D. a hard nut to crack很难解决的问题。根据空前such a bridge over the bay及空后but the local government made it within two years可知，在海湾上建造这样一座桥是一件困难的事情。故选D。