—I'm a teenager's mother. Sometimes I don't know ____.—Maybe he suffers from stress.
—Listen ! Helen is singing in the next room. It ____ be Helen. She has gone to Beijing.
The presidents of China and the USA had ________ interview a short time ago. —And it was ________ great success.
The children are having fun ________ on the playground.
We are supposed to ___________a solution to stop people from looking down at their mobile phones while driving cars.
.I was supposed ___ at 7:00 ,but I arrived at 8:00.