---Mum I’m leaving for Beijing tomorrow morning.Oh good. But be sure to phone me yo

问题补充：

---Mum, I’m leaving for Beijing tomorrow morning.

--- Oh, good. But be sure to phone me you get to Beijing, please.

A. because

B. why

C. when

D. if

答案：

A. because

B. why

(答案→)C. when

D. if

解析：连词辨析。Because因为；why为什么； when当……时； if是否，如果。句意：妈妈，我明天上午要动身去北京了。——哦，太好了。但是当你到达北京时一定要给我打电话。结合语境可知选C。