The doctor ____ the crying baby but couldn’t find out what was wrong with it.

A．looked after

B．looked over

C．looked for

D．looked out

需求是指消费者在某特定时期内和一定市场上，按一定价格愿意并且能够购买的某种商品或劳务的( )。

A．总和
B．水平
C．质量
D．数量