Scientists ______ thousands of chemicals before they decided on the right one in the end.

Only 16 percent of China’s area ____ covered withforests, so ____of us should protect trees.

Many personnel managers say it is getting harder and harder to ________ honest applicants from the growing number of dishonest ones. [ ]

A recent survey shows the number of people who shop in stores________，while far more are shopping online.

I was so tired after a day of hard work that I could______ move any longer.