3. Which is necessary for a feedback mechanism? a. control unit b. effector c. sensor d. all of these. 14. In moist soil, Terry the ameba absorbs as much water from its surroundings as diffuses out of its cell. This is an example of a. homeostasis b. dynamic equilibrium c. thermoregulation d. feedback. 15. Which structure regulates breathing rate in humans? a. diaphragm b. lungs c. hypothalamus d. medulla.