My birthday is _______ March 18. [ ]
A. at
B. in
C. on
D. to
C
方程x+2y=7有几组解，求出其正整数解．
风湿性瓣膜病时，最常受累，的：两个瓣膜是
A．二尖瓣和三尖瓣
B．二尖瓣和主动脉瓣
C．二尖瓣和肺动脉瓣
D．主动脉瓣和肺动脉瓣
E．肺动脉瓣和三尖瓣
