A green dot in the middle of 4 concentric circles. The first away from the green dot is labeled n = 1. The second circle away from the green dot is labeled n = 2. The third circle away from the green dot is labeled n = 3 and the last is labeled n = 4. There is an arrow from n = 4 to n = 1 labeled A. There is an arrow from n = 3 to n = 2 labeled B. There is an arrow from n = 3 to n = 1 labeled C. There is an arrow from n = 2 to n = 1 labeled D.