John is a consultant who issues an invoice December 18th. The company he works for always pays within ten days, including the time for the check to be mailed. If John checks his mailbox on January 1st and the check is there, which of the following tax rules would cause it to be taxable in the prior year? a. Substantial risk of forfeiture. b. Tax triads. c. Constructive receipt. d. Economic benefit doctrine.