Choose the TRUE statement.

$($a$)$ Bluetooth specifies an entire suite of protocols which is completely focused on the link layer but it provide placeholders for application protocols to be plugged into it$.$

$($b$)802\mathrm{.}11$ standards enable the creation of a link abstraction that connects one node to another in what appears to be a point$-$to$-$point manner.

$($c $)$ ARQs are mainly used to increase the performance of network systems.

$($d$)$ Consolidating reliable delivery, ordered delivery, and flow control into a single sliding window protocol is an example of Separation of Concerns $($a design principle$).$