Which statement best describes how the magnitude of the electrostatic force between two fixed point charges changes when the space between them is completely filled with a uniform material whose relative permittivity (dielectric constant) k>1?
a) It increases by a factor equal to k compared with its value in vacuum.
b) It decreases by a factor equal to k2 compared with its value in vacuum.
c) It decreases by a factor equal to k compared with its value in vacuum.
d) It becomes zero if the material is an electrical insulator.