Which car was badly damaged ?

Bill, a salesman from London, was driving past a sports car parked outside a supermarket when he saw it start to roll down the hill. Inside the car were two young girls on the passenger seat but no driver. Bill stopped quickly, jumped in front of the sports car and tried to stop it, pushing against the front of the car. Another man, who was standing nearby, got into the car and put on the hand- brake, saving the girls from injury. It was at this point that Bill noticed his own
car rolling slowly down the hill but going too fast for him to stop it. It crashed into a bus at the bottom of the hill and was so badly damaged that it had to be pulled away to a garage. As if this was not bad enough, Bill now found he had no one to blame. He was so busy chasing his car that he did not get the name of the driver of the sports car, who just came out of the supermarket and drove away without realizing what had happened.