On September 20, 2017, Greg entered into an agreement to exchange an eight-unit rental apartment building for a four-unit rental apartment building. The closing took place on October 20, 2017 at which time the transfers were completed. Gregs adjusted basis for the eight-unit building was $320,000 and the fair market value was $400,000. The fair market value of the four-unit building, which was subject to a $40,000 mortgage, was $440,000 on the date of the transaction. What, if any, is Gregs taxable gain?

a. $120,000

b. $80,000

c. $48,000

d. $40,000

e. $0