A tutoring room compares notebook bundles, counting any free notebooks. Use cost per notebook to compare: Bundle A: 6 notebooks for $9.00; Bundle B: 9 notebooks for $12.60; Bundle C: 12 notebooks plus 3 free notebooks for $19.50; Bundle D: 20 notebooks for $28.00. Which option is the best buy? Use unit rates rounded to the nearest hundredth when needed.