A Star in the Instability Strip Pulsates Because

Multiple Choice

A star in the instability strip pulsates because

A) the energy produced by the core is not constant.
B) the energy produced by the hydrogen burning shell is not constant.
C) the core of the star is composed of helium and degenerate electrons.
D) the surface temperature of the star is increasing.
E) density and opacity of the helium ionization zone changes with time.