①Because 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.

网友应该注意of后接的是复数名词pollutants。如果没有of, 则less作为限定词修饰复数名词就是语法错误了。所以这里用不定代词little的比较级less作宾语，of certant pollutants作定语。这里还需要注意：less并不是表示污染物的种类少了，而是每一种污染物的排放量少了。每一种污染物都是物质名词，不可数概念，所以每一种污染物的量都少了要用less 而不是用fewer。

②Bihar is India's poorest state, with an annual per capita income of $111, lower than that of in the most impoverished countries of the world.

这个句子中of后接in，明显是错误，不应该有任何犹豫。你此时应该核对原出处。我在百度查到的是这样的选择题：

Bihar is India's poorest state, with an annual per capita income of $111,                the most impoverished countries of the world.

A   lower than in 

B   lower than that of 

C   and lower than that of 

D   which is lower than in 

E   which is lower than it is in 

答案选B， lower than that of. 这其实是一个很简单的问题！