(1) When the current indicates Imax=0.6A, the voltmeter indicates the minimum.

According to ohm's law, the voltage across the resistor R 1 is:

u 1 = I 1r 1 = 0.6a× 10ω= 6V，

Minimum voltage on sliding rheostat:

U2=U-U 1=8V-6V=2V, so b is incorrect;

The minimum resistance of the sliding rheostat connected to the circuit is:

Rmin = U2Imax = 2v 0.6 A≈3.3ω, that is, the slider of the sliding rheostat is not allowed to slide to the far left, so A is incorrect;

Maximum power consumed by the circuit:

Pmax=UImax=8V×0.6A=4.8W, so d is correct;

(2) When the maximum voltage representation number is U2max=3V, the current in the circuit is minimum.

The voltage across R 1 is U 1'=U-U2max=8V-3V=5V,

The minimum current of the circuit is:

imin = U’ 1r 1 = 5v 10ω= 0.5A，

The range of allowable current in the circuit is 0.5~0.6A, so C is incorrect.

So choose D.