【强烈求解！GWD一道语法题！】

hermione0527

7. GWD27-Q7:
Providing initial evidence that airport are a larger source of pollution than they were once believed to be, environmentalists in Chicago report that the total amount of pollutant emitted annually by vehicles at O’Hare International Airport is twice as much as that which is being emitted annually by all motor vehicles in the Chicago metropolitan area.

A.    as much as that which is being emitted annually by all
B.    as much annually as is emitted by the
C.    as much compared to what is annually emitted by all
D.    that emitted annually by all
E.    that emitted annually compared to the

答案是D，我选的B，现在明白B错在the改变了原句的all，也知道D中that指代的是the total amount，但我想问that不是不能独立使用，后面必须加名词么？

hermione0527

自己顶一下~