问题补充：
They were busy the project the time.
A. too; to discuss; to forget
B. too; discussing; to forget
C. so; to discuss; that they forgot
D. so; discussing; that they forgot
答案：
A. too; to discuss; to forget
B. too; discussing; to forget
C. so; to discuss; that they forgot
(答案→)D. so; discussing; that they forgot
解析：句意：他们太忙于讨论这个项目以至于忘记了时间。be busy doing sth 意为忙于做某事。so……that 如此……以至于……。故选D。
