To protect the environment, the local government has put up a lot of to warn the citizens not to litter the ground.
A. sighs
B. sights
C. signals
D. signs
网友回答
A. sighs
B. sights
C. signals
(答案→)D. signs
解析：A. sighs叹息，B. sights 视野，景色，C. signals信号，D. signs标志，迹象，句意：为了保护环境，当地的政府竖立很多标志警告市民不要乱扔垃圾。根据句意选D。