$(30\%)$ Suppose two crystals are in close contact, composed of atom A and B

respectively. Their lattice structures are alike. Their atoms may be interchanged without

any change in the energy states of the crystals $($i$.$e$.$ an "ideal" solid solution$).$ Only

mixing over positions in space is considered, and not with any randomization of energy

over energy states. Suppose each crystal contains four atoms only. Please draw out all

the arrangements of the microstates of the crystals.