In a Cepheid Variable, What Happens to Make the Temperature

Multiple Choice

In a Cepheid variable, what happens to make the temperature decrease?

A) Massive solar flares release great amounts of energy, after which the star goes through a partial collapse.
B) Hydrogen fusion in the shell begins to shut down because it is running low on fuel.
C) A periodic eruption of sunspots blocks the radiation from below.
D) The ionized helium layer expands and cools.