一道关于 名词可数不可数的题

1.        Because natural gas is composed mostly of methane, a simple hydrocarbon, vehicles powered by natural gas emit less of certain pollutants than the burning of gasoline or diesel fuel.

(A) less of certain pollutants than the burning of gasoline or diesel fuel

(B) fewer of certain pollutants than burning gasoline or diesel fuel do

(C) less of certain pollutants than gasoline or diesel fuel

(D) fewer of certain pollutants than does burning gasoline or diesel fuel（E）

(E) less of certain pollutants than those burning gasoline or diesel fuel

请问为什么要用less修饰certain pollutants呢？certain pollutants在这里是不可数名词么？还是意为less amount of certain pollutants? 谢谢！！

less modifies emit.

less is adv here