In the sentence "....a simple hydroc arbon, vehicles powered by natural gas emit less of certain pollutants than those burning gasoline or diesel fuel",actually, "less" modifys the verb "emit" rather than the noun "polluants".
Just remember whenever you see such phrase as "noun. + less/more+of +verb." in a sentence, the "less/more" must modify the verb. instead of the noun.
This principle also accounts for the reason that why we don't need to use an auxiliary verb such as "do" after "than" here, because the sentence makes a comparison between two verbs, "emit more or emit less", instead of two subjects.This kind of comparison has the same meaning and effect as does the comparison between the subjects.
another instance:
Marry sings better than anyone else in her class.
As we know, we must put an auxiliary verb in the clause after "than" when we make comparison between two subjects of the clauses. The second question that you posted here, which compares the first subject "The United States government " with the other subject "any other government", is the representational example for this rule.
[此贴子已经被作者于2003-4-12 1:08:51编辑过] | 
