When a Low Mass Star First Runs Short of Hydrogen

When a low mass star first runs short of hydrogen in its core, it becomes brighter because

A) its outer, cooler layers are shed, and we see the brighter central core.
B) the helium flash increases the size of the star immensely.
C) the core contracts, raising the temperature and extending the hydrogen burning shell outward.
D) it explodes as a nova.
E) helium fusion gives off more energy than does hydrogen.