By 16:30, ______was almost closing time, nearly all the paintings had been sold.

A．which

B．when

C．what

D．that

寻呼拥塞率主要指RNC在寻呼信道（PCCH）上由于（）原因导致寻呼消息发送失败的情况。

A.资源限制

B.无线环境

C.用户未开机

D.用户忙