以下（）不属于基本业务网。
A、PSTN
B、DDN
C、PSPDN
D、IN
参考答案：D
— Can you come and play football with me?
—______. I have a lot of homework to do.
A．Excuse me
B．I’d like to
C．I’m afraid not
D．It’s a pleasure
简述过冷器的作用？
