________ on a clear day, far from the city crowds, the mountains give him a sense of peace．
A.WalkedB.To walkC.Having walkedD.When he is walking在线课程D
本题考查非谓语动词和状语从句的用法。A、B、C三项均存在与主语“the mountains”不一致的错误，故排除非谓语动词用法。D项为时间状语从句的用法。