二尖瓣脱垂者可听到( )
A．心包叩击音B．二尖瓣开瓣音C．Austin-Flint杂音D．喀喇音E．Cmham-steell杂音
参考答案：D
The young researcher laughed at some of his______colleagues who stuck to outdated ideas. [ ]
A. modern
B. present-day
C. up-to-date
D. contemporary
励磁机的正、负极性在什么情况下可能变化？
