Consider Figs. $3$ and $4$ illustrating cascade control.

$($a$)$ Suppose you were to apply feedforward control instead of cascade control to handle disturbances D and D$.$ Where do you expect feedforward control to be more beneficial: for D or D$?$ Explain why.

$($b$)$ Draw a block diagram that is a modification of Figure $15\mathrm{.}4$ that uses feedforward control of D$.$

$($c$)$ What additional sensors would be required for feedforward control of $D?$