Which of the three boys is chosen to open the train station locker with the key from inside the found leather bag and why?

Rat is the one who is chosen to open the locker with the found key since he had lived in the train station for a while and knew some of the train station workers whom he could bribe to leave him alone with the lockers. He is also a fast runner and is small, so he could lose himself within a crowd if he found he were being pursued.