“Where is Mother.” “She is in the kitchen. She _______ the housework all morning.”
| A．is doing | B．was doing |
| C．has done | D．has been doing |
Milk will turn sour soon on hot days ______ it is stored in the refrigerator．
| A．since | B．until |
| C．because | D．unless |
You'd better not ride the bicycle on snowy days; you ______ fall．
| A．must | B．might |
| C．have to | D．ought to |
—Hi, Jack．You look a bit fat now．
—Yes, I ______ five pounds recently．
| A．am putting on | B．will put on |
| C．have put on | D．had put on |
Every book ______ from the library should be returned on time．
| A．borrow | B．to borrow |
| C．borrowing | D．borrowed |
My sister has a habit of keeping a diary and writes down ______ happens every day．
| A．how | B．when |
| C．which | D．what |