问题
选择题
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
答案
答案：D
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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
答案：D