Bears ______ fat stores throughout the summer and fall to have energy enough to last them through their winter sleep.
| A．pack up | B．build up |
| C．bring up | D．take up |
-----What has made him upset recently?
-----_______ alone to face a troublesome milk case.
| A．Left | B．Being left | C．Having left | D．To leave |
----- Do you know when Tonny came back last night?
----- Yes. It was not yet nine o’clock _____ he came back home.
| A．before | B．when | C．that | D．until |
----- The windows must have taken you quite a long time that day.
----- Right. They ______ for months.
| A．haven’t been cleaned | B．didn’t clean | C．hadn’t been cleaned | D．haven’t cleaned |
It was the training he had as a young man made him such a good engineer.
| A．what; that | B．which; that | C．that; which | D．that; what |
Once harm ______ to the environment, it takes years to have the system recovered.
| A．does | B．is done | C．will be done | D．be done |