John faces a progressive tax structure that has the following marginal tax rates: 0% on the first $10,000, 10% on the next $10,000, 15% on the next $10,000, 25% on the next $10,000, and 40% on all additional income. If John earns $60,000 per year, what would be his tax liability? Select one: a. $8000 b. $9000 c. $13,000 d. $25,000