________ the danger of drunk driving, many drivers promised never to drive after drinking.
| A．Realizing | B．To have realized |
| C．Realized | D．Having realized |
________ the problem, he wouldn't have committed those mistakes.
| A．If he understood | B．Had he understood |
| C．When he had understood | D．If he would understood |
________ I be free tomorrow, I would go with you.
| A．Could | B．Should | C．Might | D．Must |
If he ________ my advice, he wouldn't have lost his job.
| A．followed | B．should follow |
| C．had followed | D．would follow |
This printer is of good quality.If it ________ break down within the first year，we would repair it at our expense.
| A．would | B．should | C．could | D．might |
Maybe if I ________ science，and not literature then，I would be able to give you more help.(2011·北京)
| A．studied | B．would study |
| C．had studied | D．was studying |