IP地址块210.161.111.1/22和201.21.151.1/21的子网掩码分别可写为

A. 255.255.252.0和255.255.248.0
B. 255.255.248.0和255.255.240.0
C. 255.255.254.0和255.255.252.0
D. 255.255.255.0和255.255.254.0