—Has your son come back from abroad for the winter vacation?—Yes. But he stayed at home for only a week. Now he      back to college.

Scientific experiments __________ by students without the teachers’ instructions can be dangerous.

To our delight, up till now, the European Union, Japan, and Russia ________ their targets for CO2 emissions (排放).

—What’s that terrible noise?—The neighbors_______for a party.

—Have you handed in your schoolwork yet?—Yes,I have.I guess it _______ now.