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
另开一贴主要是想讨论一下补出主语及do的问题。
B中可否看作是省略了vehicles？参见OG187
187. In one of the bloodiest battles of the Civil War, fought at Sharpsburg, Maryland, on September 17, 1862, four times as many Americans were killed as would later be killed on the beaches of Normandy during
D-Day.
(A) Americans were killed as
(B) Americans were killed than
(C) Americans were killed than those who
(D) more Americans were killed as there
(E) more Americans were killed as those who
有没有哪位NN可以总结一下
(1)。什么时候需要补出谓语(如这里的do)，什么时候不需要？
(2)。什么时候可以省略被比较的主语，什么时候不可以省？
简直被ETS给玩晕了。。。 | 
