When there is a fire in the place where the sensor R2 is located, the resistance of R2 decreases, resulting in a decrease in the total resistance of the circuit, so the total current in the circuit will increase. A 1 Measure the total current in the primary coil. As the current in the secondary coil increases, the current in the primary coil will also increase. Since the voltage of the power supply is unchanged and the voltage of the primary and secondary windings is unchanged, the indication of V 1 remains unchanged. With the increase of current in the secondary winding, the voltage transformation of R3 becomes larger, so the index of V2 decreases, that is, the voltage of R 1 also decreases, so the index of A2 decreases, so D is correct.
So: BC.