Two identical spheres each of radius $R$ are kept at a center-to-center spacing of $4R$ as shown in the figure. They are charged,and the electrostatic force of interaction between them is first calculated assuming them to be point-like charges at their centers $(F_c)$,and the force is also measured experimentally $(F_m)$. ($F_c$ and $F_m$ denote the magnitude of the force.)

• A
  When they carry charges of the same sign $F_c > F_m$ and when they carry charges of opposite signs $F_c < F_m$ only when they are insulators.
• B
  When they carry charges of the same sign $F_c > F_m$ and when they carry charges of opposite signs $F_c < F_m$ only when they are conductors.
• C
  When they carry charges of the same sign $F_c < F_m$ and when they carry charges of opposite signs $F_c > F_m$ irrespective of their material.
• D
  When they carry charges of the same sign $F_c > F_m$ and when they carry charges of opposite signs $F_c < F_m$ irrespective of their material.