吸气性呼吸困难，严重时可出现“三凹征”，常见病有（）
A.气管肿瘤
B.气管异物
C.喉痉挛
D.支气管哮喘
E.肺结核
参考答案：A, B, C
The boy wanted to ride his bicycle in the street, but his mother told him ______.
A．not to
B．not to do
C．not do it
D．do not to
已知A(x1,y1),B(x2,y2)，写出求直线AB的斜率的一个算法.
