A student conducts an experiment to determine Coulomb's constant, \(k\). Two small, insulated spheres carrying known positive charges \(q_1\) and \(q_2\) are placed on a track separated by a variable distance \(r\). For several values of \(r\), the magnitude of the electrostatic repulsive force \(F_E\) between the spheres is measured. Which of the following pairs of quantities, when plotted on the vertical and horizontal axes, will produce a linear graph that can be used to determine \(k\), and what does the slope of the resulting line of best fit represent?