How many of the links in the diagram in blocking mode by STP?

Click the Exhibit button to view the topology.

The diagram shows an L2 switched campus. Switch X is the STP root and switch Y is the standby
root. How many of the links in the diagram in blocking mode by STP?

A.
6

B.
7

C.
12

D.
13

E.
18

F.
19