There is an electric field at point P. A very small positive charge is placed at this point and experiences a force. Then the positive charge is replaced by a very small negative charge that has a magnitude different from that of the positive charge. Which one of the following statements is true concerning the forces that these charges experience at P? (a) They are identical. (b) They have the same magnitude but different directions. (c) They have different magnitudes but the same direction. (d) They have different magnitudes and different directions.