What is the purpose of a network mask?

What is the purpose of a network mask?

A.
It is used in conjunction with the wildcard mask to determine the number of hosts on a
network.

B.
It is used to define which parts of the IP address are allocated to host addresses and
network prefixes.

C.
It is used to hide subnets from external networks by masking the subnets with NAT.

D.
It is used to differentiate a network as a classful network or a classless network.