B, when the variable resistor R resistance value is adjusted up, resulting in a decrease in the current of the circuit, then the potential at point A rises, so B is wrong;
C, when the variable resistor R resistance value is adjusted up, resulting in a decrease in the current of the circuit, then the potential at point A rises, due to the potential at point A, φA < φ0, the control switch is in the disconnected state When φA ≥ φ0, the control switch turns on the circuit and M starts to operate. Work critical temperature did not change, but early to be connected, so C error;
D, if the metal thermistor instead of semiconductor thermistor, when the temperature rises, its resistance increases, resulting in a decrease in the potential of point A, the switch will be in the disconnected state, so it can not play the role of heat exhaust, so D error;
Therefore, A
.