Why Should the Period-Luminosity Relationship Be Different for Type I

Why should the period-luminosity relationship be different for Type I Cepheids and Type II Cepheids?

A) Type II Cepheids are dwarf stars and thus much less luminous than Type II giants.
B) Type I Cepheids are metal-rich and thus have more heavy elements in their atmospheres.
C) Type II Cepheids are metal-rich and thus have more heavy elements in their atmospheres.
D) Type I Cepheids get their energy primarily from hydrogen fusion while Type II Cepheids get their energy primarily from helium fusion.