问题补充：
I didn’t know .
A. when did the plane take off
B. when the plane took off
C. when the plane takes off
D. when does the plane take off
答案：
A. when did the plane take off
(答案→)B. when the plane took off
C. when the plane takes off
D. when does the plane take off
解析：特殊疑问词作宾语从句，将特此作为连接词，后接陈述句语序。故选B
