an beings should be very careful with what we do with nature, _____power is beyond our understanding.

Great changes __ in the city in the past 20years, and a lot of factories ____.

You’d better not phone the manager between 7 and 8 tomorrow evening; he ________ an important meeting then

He decided a great writer .

I won’t tell the student the answer to the math problem until he on it for more than an hour.