An octahedron consists of two square-based pyramids glued together along their square bases to form a polyhedron with eight faces. imagine an ant that begins at the top vertex and walks to one of the four adjacent vertices that he randomly selects and calls vertex a. from vertex a, he will then walk to one of the four adjacent vertices that he randomly selects and calls vertex b. what is the probability that vertex b will be the bottom vertex? express your answer as a common fraction.