问题
单项选择题
This area has been free from Foot—and—Mouth Disease ( ) two years.
A．for
B．in
C．after
D．before
答案
参考答案：A
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This area has been free from Foot—and—Mouth Disease ( ) two years.
A．for
B．in
C．after
D．before
参考答案：A
=2
,开关断开时，电流表示数为
,
消耗的电功率为 P1；开关S闭合时，电流表示数为I2.R2 消耗的电功率为P2.则
