题文
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 |
推荐试卷
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 |
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