For a Single-Server, Single-Line, Single-Phase Waiting Line System, Where $\lambda$ Represents the Mean Arrival Rate of Customers And

For a single-server, single-line, single-phase waiting line system, where $\lambda$ represents the mean arrival rate of customers and $\mu$ represents the mean service rate, what is the formula for the average number of customers in the system?

A) $\lambda$ / $\mu$
B) $\lambda$ / ( $\mu$ - $\lambda$ )
C) $\lambda$ ² / $\mu$ ( $\mu$ - $\lambda$ )
D) 1 / ( $\mu$ - $\lambda$ )
E) $\lambda$ / $\mu$ ( $\mu$ - $\lambda$ )