There are quite a few old books on the shelf, but _____ of them is useful to teenagers.

Give it to _____ you think can do the work well.

Protecting the earth is not ____ to be left to the government．Everybody should be concerned．

Don’t worry. You can _____he ‘ll turn up on time. He always keeps his promises.

Doctors should take special care when treating children because their breathing rate is different from _____ of adults.