What Distinguishes Interstitial Diffusion from Substitutional Diffusion

What distinguishes interstitial diffusion from substitutional diffusion?

A) Interstitial diffusion occurs only when the diffusing atom and the atoms in the lattice are the same.
B) Interstitial diffusion does not have to displace other atoms, which requires more activation energy and so substitutional diffusion is generally less probable.
C) Interstitial diffusion occurs more slowly than substitutional diffusion.
D) Interstitial diffusion requires a high enough temperature before it can take place.