joint and by-products

for the following question could you please explain why and how the answer A is derived. When I attempted this question I chose C.
A process produces two joint products A and B in equal physical quantities. A and B are sold at split off point for $5 per kg and $8 per kg respectively. There are no further costs after the split off point.

If joint costs are apportioned on a relative sales value basis, which of the following statements is true?
A Both products will have the same return on sales ratio (operating margin)
B Product A will have the higher return on sales ratio (operating margin)
C The cost per kg will be the same for both products
D The cost per kg of product A will be higher than that of product B