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
网友回答
A. when
B. while
C. because
D. that
解析：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。