Direct current, dc, flows in a direction while alternating current, ac, the direction of flow a) single, is unidirectional b) constant, periodically reverses c) opposite direction, is multiple eliminate d) periodically reversing, is constant

Consider an insulating crystal, made up of layers of atoms. what form would you expect the temperature dependence of the phonon heat capacity to approach at extremely low temperatures if the interlayer coupling is i)very strong (rigid coupling), and ii) very weak. explain.