A. When the slider P moves to the right, the resistance of the rheostat connected to the circuit decreases, and the total resistance of the external circuit decreases. According to Ohm's law of closed circuit, the total current I of the circuit increases, the terminal voltage U=E-Ir, I increases, and U decreases, so the power consumed by the lamp L decreases and will not burn out. When I 1=I-IL flows through the rheostat, if I increase and IL decreases, the voltage across the resistor R 1 increases and the power consumed by the resistor R 1 increases. The indicated value of voltmeter UV = U-U 1, U decreases and U 1 increases, so the indicated value of voltage decreases, so A is correct and BD is wrong.

C, because r=R 1, so the external resistance is greater than the internal resistance, with the decrease of resistance, the output power of the power supply increases, so c is correct.

Therefore: AC