（2014年北京市西城区高三5月二模试题）31. A person who has never tasted bitter won’t know ______ sweet is.

said he could fix my bicycle, but I really doubtit.—_______. He’s very good at this sort of thing.

_______ in this way can we save nature and animals, which is of great value to human beings

We each took ______ tea after having ______ wonderfulsupper at my aunt’s yesterday.

It took us several hours to make the old man_____ of the safety of travelling by air.[ ]