C               

题目分析：考查非谓语动词。句意 去山上游玩的游客看到一块牌子，警告他们小心。原句应是Visitors to the hills are greeted by signs which warns them to look out，sign后面跟着一个定语从句，也可换成后置定语，因为是主谓关系，用doing形式，所以选C项。          

考点 : 考查非谓语动词