He was so deep in thought，thinking of the problem ________ he knocked into the tree without noticing it.
| A．when | B．while |
| C．because | D．that |
Although it's raining, _____ are still working in the field.
| A．they | B．but they | C．and they | D．so they |
Speak to him slowly _____ he may understand you better.
| A．since | B．so that | C．for | D．because |
You'll miss the train ______ you hurry up.
| A．unless | B．as | C．if | D．until |
When you read the book, you' d better make a mark _____ you have any questions.
| A．at which | B．at where | C．the place | D．where |
We'd better hurry ______ it is getting dark.
| A．and | B．but | C．as | D．unless |