—Excuse me, may I ask you some questions? ---Sorry, I’m too busy and haven’t even a minute to________.

When Jane began to take swimminglessons,her main_______was the fear of water。

Mr．Barret doesn’t think that all bad language should be allowed．In his opinion, there are _____ that should be kept up．

Classification is a useful ______ to the organization of knowledge in any field.

I listened to Dr．Wilson's lecture about the protection of wild animals, but I was unable to grasp its key     .