Urban (城市) crowdedness would be greatly relieved if only the_______ charged on public transport were more reasonable.

We discussed the high rate of single parent family, and wondered how to explain this ______.  

Pine trees, of which there are almost one hundred _____, are found throughout the North Temperate Zone.

Most people are in favor of the _______ against smoking in public places.

Every time I returned after an     , I felt stupid because I was behind the others.