叶轮内的液体被抛出后，叶轮中心处的压力为（）
A、常压
B、低于常压
C、真空
D、高压
参考答案：B
Only after Mary read her composition the second time __________ the spelling mistake.
A．did she notice
B．she noticed
C．does she notice
D．she has noticed
物理变化（）
