Answer the multiple choice question: A single bay car wash with a Poisson arrival rate and an exponential service time has cars arriving an average of $10$ minutes apart, and an average service time of four minutes. The server utilization is:

$($a$)67$

$($b$)24$

$($c$)60$

$($d$)2\mathrm{.}50$

$($e$)40$