What is the 'TCP/IP Classful Addressing' system and which class had the subnet mask 255.255.255.0 by default?

• A.Class A had default mask 255.255.255.0; Class B had 255.0.0.0; Class C had 255.255.0.0
• B.Classful addressing divided IPv4 into: Class A (first bit 0: 1.0.0.0-126.x.x.x, default mask /8 = 255.0.0.0); Class B (first bits 10: 128.0.0.0-191.255.x.x, default mask /16 = 255.255.0.0); Class C (first bits 110: 192.0.0.0-223.255.255.x, default mask /24 = 255.255.255.0); Class D (first bits 1110: 224.x.x.x multicast); Class E (first bits 1111: 240.x.x.x experimental). Class C had the default mask 255.255.255.0
• C.Classful addressing is still used today; CIDR replaced only the routing protocols
• D.Class B had default mask 255.255.255.0 because it has the most addresses