What Causes Thermal Pulses in Low-Mass Stars

What causes thermal pulses in low-mass stars?

A) Thermal pulses are the end result of the 5-minute vibrations like those we observe on the Sun.
B) The internal structure of these low-mass stars becomes unstable as they cross the instability strip on the main sequence.
C) A helium shell flash occurs. This re-ignites the helium shell and triggers an expansion of the outer layers.
D) Spent nuclear "ash" from the helium burning shell falls onto the hydrogen-burning core, causing it to flare up.