The question that puzzled them is they can get rid of the air pollution in the area.A. ho

问题补充：

The question that puzzled them is they can get rid of the air pollution in the area.

A. how is it that

B. that how

C. what is it

D. how it is that

答案：

A. how is it that

B. that how

C. what is it

(答案→)D. how it is that

解析：考查强调句的特殊问句做表语从句：句意：那个令他们无法理解的问题是-怎么才能-他们才可以除掉这里的空气污染。强调句的特殊问句是：特殊疑问词+is it+that…，这里做表语从句，用陈述语序，所以选D。