(2013?Bengbu a mold) in the circuit shown in the figure, the negative pole of the power supply ground, R1, R2 for the constant value resistance, R3 for the sliding varistor, C for the capacitor.

A, in the sliding varistor sliding head P from the b end to the a end of the process of sliding, the varistor in the road resistance increases, the total resistance of the external circuit increases, the dry circuit current I decreases, the resistance of the voltage at the ends of the R1 decreases, then the voltage becomes smaller, so A error.

C, resistor R2 voltage U2 = E - I (R1 + r), I decrease, then U2 becomes larger, the capacitor plate voltage becomes larger, the amount of charge increases, so C is correct.

B, U2 becomes larger, the current through R2 I2 increases, the current through the ammeter IA = I - I2, I decreases, I2 increases, then IA decreases. That is, the current becomes smaller. Therefore, B is wrong.

D, according to the direction of the current in the external circuit in the direction of the current, the potential decreases, it can be seen that the potential of a is greater than zero, the potential at point a is equal to the voltage at the ends of R2, U2 becomes larger, then the potential at point a rises, so D is wrong.

The choice: C

.