there are a large number of chemical factories the air is likely to be polluted.A. That

问题补充：

there are a large number of chemical factories，the air is likely to be polluted.

A. That

B. Where

C. Once

D. As

答案：

A. That

(答案→)B. Where

C. Once

D. As

解析：where引导地点状语从句。句意：在有大量化工厂的地方，空气就很可能被污染。