= Solution
A $6$--$8M_\odot$ <main sequence> star burns hydrogen mainly through the <CNO cycle>. Its strong temperature sensitivity produces a <convective core>, which leaves a helium-rich core with a relatively sharp composition discontinuity as it retreats. After central hydrogen exhaustion, <hydrogen burning> continues in a shell while the helium core contracts and the envelope expands toward the red-giant branch.
When the core becomes hot enough, the <Triple-alpha process> starts <core helium burning>. Alpha capture on the newly made carbon also produces oxygen, so central helium exhaustion leaves a carbon-oxygen core. The star then enters the <Asymptotic giant branch> with an inert core, a helium-burning shell, a hydrogen-burning shell, and a deep convective envelope. At the high-mass end, off-centre <carbon burning> converts the carbon-oxygen core into an <oxygen-neon-magnesium core>.
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