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 |
试题篮
()
粤公网安备 44130202000953号
