How Might Kant Explain How We Distinguish Between Perfect Duties

How might Kant explain how we distinguish between perfect duties and imperfect duties?

A) No one has a duty to perfection.
B) Ought implies can.
C) Perfect duties are required by the moral law; imperfect duties are purely supererogatory.
D) An imperfect duty is one in which the maxim as universal law could be conceived without contradiction, but which we could never rationally will.