Suppose that the temperature in an exothermic continuous stirred-tank reactor is controlled by manipulating the coolant flow rate using a control valve. A PID controller is used and is well tuned. Which of these changes could adversely affect the stability of the closed-loop system?
(a) The span of the temperature transmitter is increased from 20 ^∘C to 40 ^∘C.
(b) The zero of the temperature transmitter is increased from 15 ^∘C to 20 ^∘C.
(c) The control valve trim is changed from linear to equal percentage.
(d) The feed flow rate is doubled.