A partially amortizing mortgage is made for $180,000 for a term of 30 years. The borrower and lender agree that a balance of $40,000 will remain and be repaid as a lump sum at that time. If the interest rate is 6.50%, what must the monthly payment be over the 30 year period?

a. $1,101.56
b. $1,137.72
c. $5,285.36
d. $11,700.00