$1$ Staffing at the Optimal Utilization

Time left $1$:$53$:$08$

A large grocery store is attempting to staff its check$-$out registers. Currently, the arrival rate is $=320$ customers per hour, each server has a service time of $E[T]=3\mathrm{.}75$ minutes on average, and the interarrival and service times follow an exponential distribution.

What is the distribution of the interarrival time $A?$

a$.$ A Exponential$(16)$

b$.$ A Exponential$(320$