旅客在发站办理改签时，改签后的车次票价低于原票价时，（），核收手续费。
A.退还票价差额
B.不退还差额
C.退还手续费
D.不予办理
参考答案：A
—Jack, this is the third time that you have been late this week.
—Sorry, sir, but I was ____ for 15 minutes on the way here in the traffic jam.
A．held up
B．put up
C．taken up
D．given up
样本含量n小于20、含有不完全数据的未分组资料计算生存率宜采用
A．寿命表法B．log-rank检验C．乘积极限法D．F检验E．t检验
