The average service rate at an automatic teller is $15$ customers per hour and a customer arrives on average about every $10$ minutes. On average, how many customers are in the queue at any one time?

a$.1\mathrm{.}667$ Customers

b$.0\mathrm{.}291$ Customers

c$.0\mathrm{.}914$ Customers

d$.2\mathrm{.}226$ Customers