Corey in a Desperate Need to Get to the Airport

Multiple Choice

Corey in a desperate need to get to the airport decides to take his neighbor Tanner's help. Tanner agrees to help and drives Corey to the airport. Upon arrival at the airport, Corey promises to pay Tanner $25 for the gas fill. Which of the following conditions would have made the contract between Corey and Tanner enforceable?

A) if Tanner had driven Corey to the airport in the past
B) if Corey, upon arrival, had promised to pay gas money to Tanner whenever he could
C) if the promise to pay gas money had been made before the trip was made
D) if Tanner was contracted to drive Corey to the airport as a duty