某分部工程双代号网络计划如下图所示，其关键线路有（）条。
A．2
B．3
C．4
D．5
参考答案：C
甲、乙两人绕城而行，甲绕城一周需3小时，现两人同时同地出发，背向而行，乙自遇甲后，再行4小时，才能到达原出发点，则乙绕城一周需______小时．
When I was young, I have so much time, but these days I in school all day. [ ]
A. use to, stay
B. used to, stayed
C. used to, stay
D. use to, stayed
