I _____ my homework when the door bell _____.   [ ]

A. did; rang  

B. was doing; was ringing    

C. did; was ring    

D. was doing; rang

读某城市的昼夜人口变化图。据此完成下列小题。

对图中四处功能区的判断，正确的是（）。

A、①处为商业区

B、②处为居住区

C、③处为工业区

D、④处为商住混合区