What is the main challenge when building publish$-$subscribe systems? $(2$ points$)$

$($a$)$ Deciding if the publisher is the client or the server is the main challenge of implementing publish$-$subscribe systems.

$($b$)$ Creating easy$-$to$-$use interfaces between the publisher and subscriber layers is the main challenge in implementing publish$-$subscribe systems.

$($c$)$ Naming the event$/$content and identifying$/$matching which subscriber should receive is the main challenge in implementing publish$-$subscribe systems.

$($d$)$ Properly encapsulating the resources of the subscriber as objects is the main challenge of implementing publish$-$subscribe systems.