You be hungry after the long walk. Help yourself to some cakes.
A:should
B: must
C:need to
D:ought to
____直到……时候
指从过去某一点到从句所示的时间为止的一段时间。如:
By the time we got to his house, he had finished supper.
在我们到达他就已经吃完了晚饭。
如图甲所示,矩形金属线框绕与磁感线垂直的转轴在匀强磁场中匀速转动,输出交流电的电动势图象如图乙所示,经原副线圈的匝数比为1:10的理想变压器为一灯泡供电,如图丙所示,副线圈电路中灯泡额定功率为22W.现闭合开关,灯泡正常发光.则( )