Able, Baker, and Charlie, acting independently but simultaneously, each negligently lean on Paul's car, which is parked at a scenic overlook at the edge of a mountain. Their combined force results in the car rolling over the edge of a diminutive curbstone and plummeting down the mountain to its destruction. The force exerted by each of Able, Baker, and Charlie would have been insufficient to propel Paul's car past the curbstone, but the combined force of any two of them is sufficient. Who was a factual cause of the destruction of Paul's car?