_____ do you write to your friend? — Twice a week. [ ]
Is air the lightest thing in the world?– Not . Scientists from Zhejiang University have just developed the world’s lightest material.
He is so easy-going a man _________ everyone wants to work with.
_______do you visit your grandparents?—Almost once a month.[ ]
I’m going to the States for further study. —How long ____ you ___there?