Which of the following statements are true?

(a) Feedback and feedforward control both require a measured variable,

(b) The process variable to be controlled is measured in feedback control.

(c) Feedforward control can be perfect in the theoretical sense that the controller can take action via the manipulated variable even while the controlled variable remains equal to its desired value.

(d) Feedforward control can provide perfect control; that is, the output can be kept at its desired value, even with an imperfect process model.

(e) Feedback control will always take action regardless of the accuracy of any process model that was used to design it and the source of a disturbance.