A network administrator configures the interface fa$0/0$ on the router R$1$ with the command ipaddress$172\mathrm{.}16\mathrm{.}1\mathrm{.}254255\mathrm{.}255\mathrm{.}255\mathrm{.}0$However, when the administrator issues the command show ip route, the routing table does not show the directly connected network. What is the possible cause of the problem? a$.$ The IP address is not a valid host b$.$ No packets with a destination network of $172\mathrm{.}16\mathrm{.}1\mathrm{.}0$ have been sent to R$1$ c$.$ The administrator needs to save the configuration first d$.$ interface fa$0/0$ is shutdown