7.Although the causes of cancer ____， we do not yet have any practical way to prevent it.  

—What's that noise?—Oh，I forgot to tell you.The new machine ________.

They say the snow will ________ until the end of next week. 

What shall we use for power when all the oil in the world has        ?

5.---Have you handed in your schoolwork yet? 　---Yes，I have. I guess it ____ now.