We can never expect ______ bluer sky unless we stop______ industrial waste going into _____ atmosphere.

What were you up to when yourparents came in— I ____ for a while and ____ some reading

My mother always tells me not to forget to ________ my umbrella when I leave for work.

Many died or returned home，but most remained in California to _____ a life for themselves in the new towns or on farms.[ ]

The scientist decided to devote his life to ____ pollution from ____ again.