To protect the environment the local government has put up a lot of to warn the citizens

问题补充：

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

答案：

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。