Scientists believe native Americans arrived by crossing the land bridge that connected Siberia(西伯利亚)and more than 10,000 years ago.
______ good, these cookies sell well.
His parents were so poor that they couldn’t _______ to buy a computer for their only son .
Who did the teacher , as well as the monitor , _______ an article for the wall newspaper?
The question _______ we can get enough coal for the winter has not been cleared up yet .
Every possible means ________ to prevent the air pollution , but the sky is still not clear.