— I’m sorry that John is out.— Please ask him to call me as soon as he __________.

The traffic is moving very slowly as so many cars   on their way back to Shanghai.

I think that ____ very difficult to learn.

According to the recent regulation, it is required that any chemical plant that_______ along the river _______ to build a recycling center for the waste.

The teacher， together with the students, _ __ discussing Reading Skills that ______ newly published in America.