(2) The mass ratio of carbon, hydrogen and oxygen in vitamin D2 is (12× 28): (1× 44):16 = 84:1:4.
(3) Calcium oxide and water generate calcium hydroxide and give off a lot of heat. Although calcium oxide contains calcium, it cannot be used as a calcium supplement. Calcium carbonate needs to be added with other substances to be used as calcium supplement; Calcium gluconate is suitable for direct administration as a calcium supplement.
So the answer is: (1) 396; (2)84: 1 1:4; (3)C。