$27)$A post office has three clerks at the service counter and a single waiting line. Whenever a clerk becomes free, he$/$she waits on the customer at the front of the waiting line. This is an example of a$($an$)\_\_\_\_\_\_\_\_\_$ queue system.

A$)$pooled

B$)$separate

C$)$single$-$server

D$)$inflexible