What is the average response time equal to on a queued system with random arrivals and random respon

What is the average response time equal to on a queued system with random arrivals and
random response times (S is the server’s service time and U is its utilization)?

A.
U/(1-U)

B.
US/(1-U)

C.
S/U(1-U)

D.
S/(1-U)