As astronomers extend their distance ladder beyond 30 Mpc, they change their measuring standard from Cepheid variable stars to Type Ia supernovae. Why is this change necessary? Choose one: A. Type Ia supernovae vary more slowly than Cepheid variables. B. Type Ia supernova vary more quickly than Cepheid variables. C. Type Ia supernova are less luminous than Cepheid variables. D. Type Ia supernovae are more luminous than Cepheid variables