The plaintiff was travelling in a bus belonging to the State Road Transport Corporation resting her elbow on the windowsill. The bus was moving on the highway outside the town area. A truck coming from the opposite direction hit her elbow as a result of which she received severe injuries. The bus and the truck, however, did not come in contact with each other. The plaintiff claimed damages against the drivers of the bus as well as the truck. On which one of the following grounds were the defendants held liable, if any?