----Must I do well in all the subjects, Mum?----Sure, baby! As an old saying goes, knowledge is _________.

Father went to his doctor for＿＿＿＿＿about his heart trouble．

Thanks is a common _____ used in English. You can never use it too much.

In the time of social reform, people’s state of mind tends to keep       with the rapid change of society.

According to news reports, personal ____ of guns in the USA causes a lot of trouble.