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.
(c) less of certain pollutants than gasoline or diesel fuel.
(e) less of certain pollutants than those burning gasoline or diesel fuel. 正確答案E中的those到底指代上文的什么東西？ | 
