An experienced astronomer can take one look at the H-R diagram of a star cluster and immediately estimate its age. Select the statement that explains how this is possible. Choose one: A. The total number of stars in the cluster indicates its mass; greater mass means heavier elements are present. The more heavy elements there are, the older the cluster is. B. The most massive stars remaining on the main sequence indicate the cluster's age. C. Older stars are more luminous, so the more luminous a star is in the cluster, the older it is. D. One of the axes of the H-R diagram is age