原发性免疫缺陷病的总发病率是（）
A.1∶100000
B.1∶50000
C.1∶10000
D.1∶5000
E.1∶2500
参考答案：C
下列说法中：①过两点有且只有一条直线，②两点之间线段最短，③到线段两个端点距离相等的点叫做线段的中点，④线段的中点到线段的两个端点的距离相等。其中正确的有 （ ）
A．1个
B．2个
C．3个
D．4个
They ________ have arrived at lunchtime but their flight was delayed. [ ]
A. will
B. can
C. must
D. should
