What is the difference between co-products and by-products?

What is the difference between co-products and by-products?
A . Only co-products appear in the process or production order settlement rule.
B. Only co-products can be manufactured in isolation from the main product.
C. Only co-products are identified by a special item category in the bill of material (BOM).
D. Only by-products have a zero inventory valuation.

Answer: A