左心室特点（）
A、入口周缘有二尖瓣
B、左上部为动脉圆锥
C、出口为肺动脉口
D、前内侧部为主动脉前庭
E、有节制索
参考答案：D
Studying in groups is necessary_____ you want to do well in school.
A．if
B．until
C．unless
D．though
干沉降
