What are the correct descriptions of Rings Mains?

A、Usedindistributionsystemforareasupplies.
B、Useslesswireusedthanparallelfeeders.
C、Continuityofsupplyismaintainedifonefeederislost(Hotbackup).
D、Alsousedinsub-transmissionsystemsforlightlyloadedruralareas.