What is the main challenge when building structured peer$-$to$-$peer systems? $(2$ points$)$

$($a$)$ The main challenge in peer$-$to$-$peer systems is to place components.

$($b$)$ The main challenge in peer$-$to$-$peer systems is to match events to components.

$($c$)$ The main challenge in peer$-$to$-$peer systems is to locate resources.

$($d$)$ The main challenge in peer$-$to$-$peer systems is to provide virtual resources.