Corey desperately needs to get to the airport and decides to take his neighbor T

Corey desperately needs to get to the airport and 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 towards gas money. Which of the following 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

This is for my business law class, anything will help