_____there are a large number of chemical factories,the air is likely to be pollutedA that B where C once D as
问题描述：
_____there are a large number of chemical factories,the air is likely to be polluted
A that
B where
C once
D as
答
D as作为连词时有“由于, 因为”的意思，句子意思是由于很大数量的化工厂存在，空气受到污染~~而once是一旦得意思，很明显跟题意不符，where是关系副词，放于句首不符合句意，而that 更不符合了。
答
选D
as表原因，意思是:因为；由于
答
应选B,因为本句的意思是“哪里有很大数量的化工厂,那里的空气可能就被污染”所以根据句意,应选B