Consider a single$-$server service system with average customer arrival rate of $$customers$/$minute and average service rate of $$ customers$/$minute$.$ Assume that both the interarrival$$times and the service times are exponentially distributed. The average total time a$$customer spends in the system $($in minutes$)$ is given by which of the following expressions?$$a$.!"$#$"$$#$$b$.$ #$"$$#$$c$.!"$d$.!"$$#