4. Which one is correct in the following description about diffusion: (A) interstitial diffusion is usually not as fast as vacancy diffusion； (B) interstitial atoms are smaller, so their mobility is less； (C) copper-nickel interdiffusion belongs to interstitial diffusion, (D) carbon-iron interdiffusion belongs to interstitial diffusion, and (E) interstitial diffusion cannot occur at concentration equilibrium.