---Is this key ________?--No, it isn’t. It’s ___________.
—Is this _____ schoolbag? —No. It's _____.
Not only oral English but also written English _____ important.
—We’ll make a trip to Hainan Island next weekend. Will you go with us?—No, I can’t _____ it at present.
—How many students are there in the classroom?—_____ I don’t know where they have gone.
The rubbish _____ is produced every day is becoming a serious problem in cities around the world.