(2006? Mianyang) As shown in the figure, when the switch S is closed, the lamp L emits light, and the slider P of the sliding rheostat slides () A to the left, indicating the current.

As can be seen from the figure, the sliding rheostat is connected in parallel with the bulb, and when the slider P of the sliding rheostat slides to the left, the resistance of the sliding rheostat becomes smaller;

According to the relationship between parallel resistance and total resistance, it can be seen that the total resistance of parallel part becomes smaller;

If the resistance of R2 remains the same, the total resistance in the circuit becomes smaller;

According to ohm's law I=UR, it can be known that the total current increases when the total voltage is constant.

In addition, the resistance of R2 is constant,

∴ According to U2=IR2, U2 will become larger;

In addition, the power supply voltage is unchanged,

∴UL=U total -U2, and then UL becomes smaller;

According to P=U2LR, we can know that the actual power of the light bulb will become smaller;

The light bulb will dim.

So choose D.