A. According to the circuit diagram, the constant resistor is connected in series with the sensor resistor. When the concentration of carbon monoxide increases, RCO decreases, and the voltage between bc is equal to the voltage across the varistor. Ubc=I？ R=E？ RR+RCO？ ① increase. Therefore, the potential at point B increases. So, A is wrong;

B, c, obtained from formula ①: UBC = e 1+rcor? ② To improve the sensitivity of gas sensor, the following methods can be adopted: increasing the electromotive force of power supply (the voltage between alternating currents) or increasing the resistance value of rheostat R, therefore, B is wrong and C is correct;

D, from ②, it can be seen that if the voltage between AC decreases, the resistance of R needs to be increased to make the gas sensor work normally, so D is wrong.

So choose: C.