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。