Answer the multiple choice question: A multiple server queuing system with a Poisson arrival rate and exponential service time has an average arrival rate of $4$ customers per hour and an average service time of $18$ minutes per customer. The minimum number of servers required to avoid an overloaded system is:

$($a$)3$

$($b$)4$

$($c$)2$

$($d$)5$

$($e$)1$