$4\mathrm{.}1-1.$ The network layer $-$ where is it$?$ Check all of the statements below about where $($in the network$)$ the network layer is implemented that are true.

Select one or more:

a$.$ The network layer is implemented in routers in the network core. $.$

b$.$ The network layer is implemented in wired Internet$-$connected devices but not wireless Internet$-$connected devices.

c$.$ The network layer is implemented in Ethernet switches in a local area network.

d$.$ The network layer is implemented in hosts at the network's edge.