The Petersen graph P can be defined as follows. Its vertices are the 2-subsets of {0, 1, 2, 3, 4}, and two subsets S_1 and S_2 are endpoints of an edge in P if and only if S_1 intersection S_2 = (a) How many vertices does the Petersen graph have? (b) Show that the figure below represents the Petersen graph by correctly labeling its vertices as 2-subsets of {0, 1, 2, 3, 4}