Broker Billy Secured a Written Listing from a Large Company

Broker Billy secured a written listing from a large company which owned an unused warehouse. Billy later entered into a verbal agreement to share his commission with cooperating broker, Cynthia. The cooperating broker procured a buyer, but later Billy refused to split the commission at closing. Cynthia could

A) sue to recover her share.
B) not sue because this was only a verbal agreement.
C) ask the real estate commission to arbitrate.
D) forbid her sales agents from ever working with Billy's agents in the future.