A rhombic dodecahedron is a convex polyhedron where each of the 12 faces is a rhombus, and all of the faces are congruent to each other. The number of edges that meet at a vertex is either 3 or 4 , depending on the vertex. What is the number of vertices at which exactly 3 edges meet?
Answer Choices
A. 5
B. 6
C. 7
D. 8
E. 9