—Do you know?—Yes. At about twelve. I was just about to go to bed. A. when he came backB.

问题补充：

—Do you know?

—Yes. At about twelve. I was just about to go to bed.

A. when he came back

B. when he will come back

C. when did you go to bed

D. whether he goes to bed late

答案：

(答案→)A. when he came back

B. when he will come back

C. when did you go to bed

D. whether he goes to bed late

解析：考查宾语从句。从句为特殊疑问句，常选择what,when,where,which,who,how等的疑问代、副词作连接词时，语序要用陈述语序。根据答语是一般过去时，另根据后句句意为“是的。大约在十二点。我刚要睡觉。”故选A。