A : Entropy of a perfect crystalline substance at absolute zero is zero. 

R : At absolute zero, the translational kinetic energy of a system is zero.

• Enthalpy and entropy of any elementary substance in their standard states are taken as zero because, at absolute zero, particles of the perfectly crystalline substance become completely ordered.
• The entropy of a perfect crystalline substance at absolute zero is zero. At absolute zero, the translational kinetic energy of a system is zero.

So, Both A and R are true, and R is the correct explanation.