The average service rate at an automatic teller is $18$ customers per hour and a customer arrives on average about every $4$ minutes. On average, how many

customers are in the queue at any one time?

a$.0\mathrm{.}833$ customers

b$.5\mathrm{.}000$ customers

c$.4\mathrm{.}167$ customers

d$.5\mathrm{.}833$ customers