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

For a single-line, multi-server, single-phase waiting line system, where $\lambda$ represents the mean arrival rate of customers and $\mu$ represents the mean service rate for each of s identical servers, what is the formula for the average utilization of the system?

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