A, when the light is enhanced, the resistance of the photoresistor decreases, so the output power of the transformer increases, so the input power also increases, so A is correct;

B, when the sliding contact P slides upwards, the number of turns of the primary coil increases, and the output voltage of the transformer decreases according to the direct ratio between the voltage and the number of turns; According to the power expression P=U2R, the output power decreases, so the input power decreases, so b is correct;

C, when u increases, according to the voltage is proportional to the number of turns, the transformer output voltage increases; According to the power expression P=U2R, the power consumed by electrical appliances increases; Therefore, C is correct;

D, when F decreases, the input voltage and output voltage are unchanged, and the output power is unchanged, so the input power of the transformer is unchanged, so D is wrong;

Therefore: ABC ..