If the source resistance of the signal source is 1kohm，and when the amplifying circuit is not connected,the voltage amplitude of the signal source is 10mV， but when the amplifying circuit is connected,the voltage of the input port of the amplifying circuit is 8mV， then the input resistance of the amplifying circuit is ( )k ohm.A:5 B:4 C:2 D:1 E:0.5 答案: 4