问题
选择题
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
解析 考查状语从句的引导词。so...that...表示“如此……以致……”，引导结果状语从句。句意为：他思考问题太投入了，以致一不留神儿撞到了树上。