A solar-mass star's motion up the subgiant & red-giant branches is similar to its motion up the asymptotic giant branch in that:?
A. The core (helium and carbon, respectively) is not burning and consequently contracts.
B. The core heats the burning shell around it (hydrogen and helium, respectively), which increases its burn rate and consequently causes the outer layers to expand and cool.
C. The core eventually contracts to the point that electron degeneracy pressure sets in.