So difficult __________ it to work out the problem that I decided to ask Tom for advice.
| A．I did find | B．did I find |
| C．I have found | D．have I found |
It is strange that the small boy _________ work out such a difficult exercise, which is even hard to settle for adults.
| A．should | B．might | C．could | D．would |
His wife has left him, but he behaved _______ nothing had happened.
| A．even though | B．as soon as | C．as though | D．as long as |
Clearly, in a system _________ every decision is made on the basis of tradition alone, it may be difficult to achieve progress.
| A．that | B．when | C．where | D．which |
Suddenly it _________ to me that I should report the accident to the police.
| A．happened | B．struck | C．occurred | D．hit |
The following chapters provide step-by-step instructions for practical _______ of genetic (基因)engineering to agriculture.
| A．applications | B．organization | C．expression | D．attention |