怎么样才能最有效地看出比较的是宾语还是主语？

请各位大牛介绍一下最直接的方法，区分比较句型中比较的是宾语或者是主语，有时候老太清楚，迷糊！

例如：

16.   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

这题我就认为比较的是主语，可看答案好像是比较的是宾语

比较的是vehicle,

vehicles powered by natural gas 与vehicles burning gasoline or diesel fuel进行比较