8. 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 |
相关知识点
推荐套卷
8. 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 |