What does entering the system mean

Why do the strengths of london (dispersion) forces generally increase with increasing molecular size? choose one: a. heavier atoms have stronger attractions for each other than lighter atoms. b. dispersion forces are all equal in magnitude; there is no size dependence. c. dispersion forces arise from the attraction between the nuclei of atoms, and larger molecules have larger nuclei. d. dispersion forces arise from dipoles caused by the electron distribution being distorted. larger molecules have more electrons and, therefore, more distortions and a bigger force. e. dispersion forces depend on distance. larger molecules are farther apart and so the forces are smaller.