Answer ABD, do this: the sum of the voltages of all the appliances in the series circuit is equal to the power supply voltage, which remains unchanged. The voltage obtained by ohm's law is equal to the current multiplied by the resistance. Because the resistance of the sliding rheostat increases and the circuit current decreases, the voltage of R decreases, so the voltage of the rheostat increases (because the sum is unchanged), and P=UI, so the power of R decreases. As for the power of sliding rheostat P = ei-rii, I was changed from 0.6 to 3/25. According to the knowledge of quadratic function, the power of sliding rheostat first increases and then decreases.

The last option of this question is beyond the scope of junior high school mathematics, so it is more difficult to do!