在发展中国家，二尖瓣关闭不全的最常见病因是（）。
A.二尖瓣脱垂
B.风湿性心脏病
C.感染性心内膜炎
D.二尖瓣环钙化
E.冠心病
参考答案：B
Tony has to go to school by bus because his bicycle .
A．will be repaired
B．is repaired
C．has been repaired
D．is being repaired
设计一个方案，测量你自身头发所能承受的最大拉力，说明你的设计方案和测量步骤．
