Art invested $100 two years ago at 8 percent interest. The first year, he earned $8 interest on his $100 investment. He reinvested the $8. The second year, he earned $8.64 interest on his $108 investment. The extra $.64 he earned in interest the second year is referred to as:1. Free interest.2. Bonus income.3. Simple interest.4. Interest on interest.5. Present value interest.