— What's the matter with Jane? She has b
题型：选择题
问题：
— What's the matter with Jane? She has been sitting there for hours.
— A letter from her mother _____ an attack of homesickness in her.
A. set out
B. set off
C. sent off
D. sent up
题型：选择题
| 如图为理想变压器原线圈所接交流电压的波形．原、副线圈匝数比n1：n2=10：1，串联在原线圈电路中电流表的示数为1A，下列说法正确的是（ ） A．变压器输出端所接电压表的示数为20 B．变压器的输出功率为200W C．变压器输出端的交流电的频率为50Hz D．穿过变压器铁芯的磁通量变化率的最大值为20  |
V
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