比较时助动词尽量补出

4. 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. (C-p8-16)

本句没有补出，是否说明也可以不补出，当然补出更好？

是的，补出更好，但千万别补错了。