Answer:the sensor or receptor
Explanation: homeostasis is the process of the body to maintain a constant internal condition , regardless of changes in it's external environment.
Homeostatic usually involves a sensor/receptors on which detects changes or stimuli,A control center which process the stimuli and triggers the effector and An effector which changes the body according to the message recieved from the control center.
In an air conditioner,the thermometer is the sensor,which detects a change in the temperature of the room.this allows the air conditioner to control the temperature and keep it constant, regardless of the temperature outside.
