what is the CIDR block?

Given the CIDR address range of 200.165.160.0 to 200.165.191.255, what is the CIDR
block?

A.
200.165.160.0/17

B.
200.165.160.0/18

C.
200.165.160.0/19

D.
200.165.160.0/20

E.
200.165.160.0/21