For two identical positive point charges held fixed in space, the curve of electric potential energy U versus separation r decreases monotonically as r increases. Which statement is most consistent with the relationship between the slope of that curve at a point r and the electrostatic force on either charge at that separation?
a) The slope equals the magnitude of the force, and a negative slope signifies attraction that pulls the charges together.
b) The magnitude of the force equals the absolute value of the slope, and a negative slope indicates repulsion acting to increase r.
c) Because the force acts along the slope, a negative slope results in a repulsive force that pushes the charges farther apart.
d) Since F=−q1dU/dr, the magnitude of the force equals the absolute value of the slope divided by the charge, and the negative slope therefore corresponds to attraction.