Did you write to Grace last summer ? ——No, but I’ll her over Christmas vacation

There will be less pollution _____ more people use public transportation.

Where _____ — I got stuck in the heavy traffic. I _____here earlier.

Is some American friends visited last week?

How much is _______ for a schoolbag?-Experts say students should carry_______ more than 10 or 15 percent of their body weight．[ ]