The Main Difference Between Classical Cepheids and RR Lyrae Stars

Multiple Choice

The main difference between classical Cepheids and RR Lyrae stars is

A) their masses.
B) that Cepheids form at much greater distances from Earth.
C) that RR Lyrae were discovered much earlier than classical Cepheids.
D) their pulsation mechanisms.
E) that classical Cepheids obey a period-luminosity relation, but RR Lyrae do not.