Mrs Smith has a cute baby. It often smiles at people _________. 

most of the children in my time liked to play together under that big tree.___ during the summer holidays

 ---Have you ____ cooked for your parents?   ---Yes, of course.

-- Is air the lightest thing in the world?-- Not            . Scientists from Zhejiang University have just developed the world’s lightest material.

---If there are _______ people driving, there will be _______ air pollution.---Yes, the air will be fresher.