(a) 8 glucose molecules are broken down fully with oxygen.

Step 1. Oxygen used. Each glucose needs $6$ oxygen molecules: $8 \times 6 = 48$ molecules.

Step 2. Carbon dioxide, aerobic. Each glucose gives $6$ carbon dioxide molecules: $8 \times 6 = 48$ molecules.

Step 3. Carbon dioxide, yeast. Each glucose gives only $2$: $8 \times 2 = 16$ molecules.

Step 4. Compare. $\dfrac{48}{16} = 3$. The yeast leaves most of the carbon in the ethanol.