In the diagram above, nodes B, C, and D are sending messages to A with the indicated Source Port and Destination Port numbers. From this diagram, infer how many sockets each of the four machines have opened: (I need help understanding how they got the answers below)

a. if all nodes are using UDP ?

A: 2, B: 2, C: 2, D: 1

b. if all nodes are using TCP?

A: 5, B: 2, C: 2, D: 1

c. If B is using TCP but C and D are using UDP?

A: 4, B: 2, C: 2, D: 1

d. If B is using UDP but C and D are using TCP?

A: 4, B: 2, C: 2, D: 1

SP: 1234 SP: 6642 DP: 5678 DP: 5678 SP: 1234 SP: 6642 DP: 5678 DP: 8877 SP: 1234 DP: 5678 SP: 1234 SP: 6642 DP: 5678 DP: 5678 SP: 1234 SP: 6642 DP: 5678 DP: 8877 SP: 1234 DP: 5678