$73.$ A bank always has two tellers on duty. Customers arrive to receive service from a teller at a mean rate of $40$ per hour. A teller requires an average of $2$ minutes to serve a customer. When booth tellers are busy, an arriving customer joins a single line to wait for service. Experience has shown that customers wait in line an average of $1$ minute before service begins. What is the average waiting line in the bank?

$1$ minute

$2$ minute

$3$ minute

$4$ minute

None of the above