Controlling water-pollution is necessary_____ it will be expensive.

一 Do you mean we are almost running out of food?
一 Yeah.        the canned and frozen foods are used up

Do ________ you think is right _________ difficulties you may have.

— I phoned you at about three this afternoon, but no one answered.
— Oh, that was probably _______ I was doing exercise outdoors.

We should try every effort to prevent violence from happening at school, ______ more and more students would drop out of school ______ their personal safety could not be guaranteed.

_____ he is very busy, he spends at least two hours every day surfing the Internet in order to find out the latest development in science and technology.

Several weeks had gone by _____ the head of the gallery realized the painting was missing.

Students can easily fall behind in their studies ____ they don’t manage their spare time carefully.

He warmly congratulated them on _____ they had achieved in science and technology.

It is not until recently ______ the scientists have found proof of there being water on Mars.