Final answer:
To find the net force between two opposing forces, the smaller force is subtracted from the larger one. In this case, it is 8 N towards Mary, making answer C) correct.
Step-by-step explanation:
To calculate the net force exerted on the books by Mary and Carla, it's important to know the direction of each force. Assuming they are exerting force in opposite directions (for instance, they are engaged in a tug-of-war with the books), you subtract the smaller force from the larger force to find the magnitude of the net force. Since Mary's force is 221 N and Carla's force is 213 N, the net force is 221 N - 213 N, which equals 8 N. The net force will be in the direction of the larger force, which in this case is Mary's. Therefore, the correct answer is C) Net force is 8 N, books fall towards Mary.