Questions re: $802\mathrm{.}11$ MAC $($Sec$.7\mathrm{.}3\mathrm{.}2)$

$($a$)$ Briefly describe the hidden node problem associated with wireless networks.

$($b$)$ The $802\mathrm{.}11$ MAC protocol includes a scheme to help avoid collisions and the hidden node problem. What two special control frames are introduced by this scheme and what is their purpose?

$($c$)$ Briefly explain $/$ show how the above scheme is implemented in the $802\mathrm{.}11$ CSMA$/$CA protocol using a timing diagram like the one shown below. Assume that A is transmitting to node B$.$