Why Does the Cepheid "Standard Candle" Have Limited Usefulness Beyond

Why does the Cepheid "standard candle" have limited usefulness beyond 100 Mpc?

A) Distant galaxies are seen in the past, when Cepheid variables behaved differently than they do today.
B) Distant galaxies are so young they do not contain Cepheids.
C) Cepheids are too faint to be seen beyond that distance, even with HST.
D) The light variability of Cepheids diminishes with distance so they do not appear to vary there.
E) Older distant Cepheids are of population II, where the period-luminosity relation no longer works.