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 waiting in line?

A) $\lambda$ / $\mu$
B) $\lambda$ / ( $\mu$ - $\lambda$ )
C) pL Ref pg 66 in PDF
D) 1 / ( $\mu$ - $\lambda$
E) $\lambda$ / $\mu$ ( $\mu$ - $\lambda$ )