梨形心最常见于（）
A．主动脉瓣关闭不全
B．肺动脉瓣关闭不全
C．二尖瓣狭窄
D．二尖瓣关闭不全
E．三尖瓣狭窄
参考答案：C
--- Oh, there is someone knocking at the door.
--- __________ must be your neighbour, Mrs Black.
A．There
B．She
C．He
D．It
简述卵裂的特点。
