Because natural gas is composed mostly of methane, a simple hydrocarbon, vehicles powered by natural gas emit less of certain pollutants than those burning gasoline or diesel fuel.
为什么those前面不加do呢？
In an assessment of the problem faced by rural migrant workers, the question of whether they are better off materially than the urban working poor is irrelevant.
为什么the urban前面不加are呢？
例如：The guiding principles of the tax plan released by the Treasury Department could have even greater significance for the economy than do the particulars of the plan.
