ABCBiG_Bang is a retail store having millions of purchase records every day. A customer can buy many products and a product can be purchased by many customers. Purchase details are available in a separate relation.

Customer (CustId, Name, Address)
Product (ProdId, Desc, Price, ProdValidity)
Purchase (ProductNo, Quantity, SalesAmount, CustomerNo)

Identify the Primary key for Purchase relation.

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