in the 1980's, the rate of increase of minority population in United States was twice as fast as what it was in 1970's (对) vs in the 1980's, the rate of increase of minority population in United States was twice as fast as it was in 1970's (错)
为什么这里比较句子不可以阿？ 还有
vehicle powered by natural gas emit less of pollutants than those burning gasoline and diesel fuels (对)
vehicle powered by natural gas emit less of pollutants than does burning gasoline and diesel fuels (错) 一里面为什么可以不加does 作谓语。
还有
dirt roads cost twice as much to maintain as do paved roads 这里为什么又是比较从句了呢，一头雾水，
