Marginal consumer surplus falls from third to fourth can Steps:

• Surplus on third can: $6 (WTP) -$4 (price) = $2.
• Surplus on fourth can: $4 (WTP) -$4 (price) = $0.
• Change in marginal surplus: $0 -$2 = -$2.
• Buying fourth thus lowers marginal consumer surplus by $2.

Why A is correct:

• Marginal consumer surplus, the gain on the additional unit, drops by exactly $2 per the definition of surplus as WTP minus price.

Why the others are wrong:

• B: Total surplus across all cans stays $18, but question targets effect on surplus from buying the fourth (marginal).
• C: $4 equals price paid, not surplus change.
• D: $3 is WTP for irrelevant fifth can.

Final answer: A