Now the air in our city is than it used to be. Something must be done to stop it.A. goodB

问题补充：

Now the air in our city is than it used to be. Something must be done to stop it.

A. good

B. bad

C. worse

D. better

答案：

A. good

B. bad

C. worse

D. better

解析： good形容词，好的；bad形容词，坏的；副词，很，非常，坏地；worse更坏的，更糟糕的；better更好的。联系下文，必须做点什么来组织这种情况。可知前文指的是，现在我们城市的空气比过去差多了。故选Ｃ。