CREATE TABLE Purchaser{ 
      name string,
      credit_card integer (PRIMARY KEY), 
      address string 
}
CREATE TABLE Product{ 
    product_id integer (PRIMARY KEY), 
    product_name string 
}
CREATE TABLE Purchaser_Product{ 
    credit_card integer, 
    product_id integer, 
    purchase_date date 
}

Based on the sample code above, what is the primary key in the Purchaser_Product table?