The company you work for currently has ISDN $($Integrated Services Digital Network$)$ service in place. Your supervisor wishes to know what sort of ISDN service is being used so that better alternatives can be considered. He tells you that the service has two B$-$Channels and a D$-$Channel. What ISDN format do you have in place?

a$.$

PRI $($Primary Rate Interface$)$

b$.$

BRI $($Basic Rate Interface$)$

c$.$

BRE $($Basic Rate Extended$)$

d$.$

POTS $($Plain Old Telephone Service$)$