How Is a Purchase Order Different from a Bill

How is a Purchase Order different from a Bill?

A) Purchase Order - records a fulfilled order of purchased products from a vendor. Bill - mailed by the vendor once the order has been approved by a company.
B) Purchase Order - record of an order to purchase products from a vendor. Bill - received when a purchase order has been fulfilled and received by a company.
C) Purchase Order - preliminary pricing of items ordered. Bill - received when a purchase order has been fulfilled and received by a company.
D) Purchase Order - record of an order to purchase products from a vendor. Bill - record collection of the customer's payment.