You have just advance refunded $50 million of outstanding tax-exempt revenue bonds with $40 million of new debt, which created a gain after considering reimbursement effects of $5.0 million. What could account for this?

a. Higher interest rate on the refunding issue

b. Lower interest rate on the refunding issue

c. Positive arbitrage on the refunding issue

d. None of the above

please show solutions and explanations.