若复数(1+2i)(1+ai)是纯虚数，(i为虚数单位)，则实数a的值是     ．

Speaker A: Do you think you could keep the noise down a bitSpeaker B: ______

A．Yes, of course I could. I will turn it down a bit.

B．Oh, I beg your pardon. I think so.

C．Yes, sorry. Am I disturbing you

D．Sorry! Have I been keeping you awake