(25 marks) Consider the following ER-diagram and assumptions for a selling system. gname Group area Belong to sid Salesman pid name Sell Product pname cid Customer producer name Every group has a unique gname. Every salesman has a unique sid. Every customer has a unique cid. Every product has a unique pid. Every product can be uniquely identified by its pname and producer. A salesman belongs to exactly one group. A group contains at least one salesmen. A salesman can sell any number of products to a customer. a) (5 marks) List all the superkey(s) of Product". b) (4 marks) List all the candidate key(s) of "Product". c) (6 marks) Assume that pid is a primary key, complete the ER-diagram by adding all missing constraints (weak entity, key constraints, participation constraints and relationship constraints). d) (10 marks) This ER model does not allow a salesman to sell one product to the same customer twice. Please briefly state why this is happening and try to modify the diagram to make it work