___ years ago, the population of China was over 20 million.

When will Han Han’s new book ________? —Sorry, I don’t know. I’m looking forward to his new book, too.

The postman shouted, “Mr Green, here is a letter ______ you.” [ ]

Teachers should ___them to continue studying when some students want to give it up.

Millie is getting on well with her friends but she is a little selfish _____.