I can’t remember when exactly the Robinsons left _____ city. I only remember it was ______ Sunday.

A．the; a

B．a; the

C．a; a

D．the; the

如图所示，闭合开关后，灯泡不亮；当玻璃珠被烧到红炽状态时，灯泡逐渐变亮．这一现象表明______．