牵引发电机的理想外特性由（）三部分组成。
参考答案：电压限制线、恒功率曲线、电流限制线
Frank , with his two sisters , _____ London by train which _______ at 8:30 next morning .
A．is leaving for ; will leave
B．are leaving for ; leaves
C．is leaving for ; leaves
D．will leave for ; will leave
用绳子测量井深，把绳子三折后，井外多出4米，把绳子四折后，井外多出1米，则井有( )米深。
A．8
B．16
C．24
D．32
