The retired teacher is said to have returned from the USA, where he ______ by some of his old students

If there are ____trees, the air in our city will be____cleaner.

The question is _____it is worth doing.[ ]

Take care, ______ you will make a lot of mistakes in this paper.

I have kept that portrait _____ I can see it every day, as it always reminds me of my university days in London