$4.$ Page $4$ of this handout contains three tables. One contains data on suppliers, one contains data on parts, and one contains data on shipments. The SHIPMENT table contains data on parts shipped by suppliers. For example, supplier S$1$ ships a quantity of $300$ units of part P$1.$ Likewise, supplier S$1$ ships a quantity of $200$ units of part P$2.$ The following six questions pertain to these three tables. $4.$ Describe the nature of the referential integrity constraint$($s$)$ that involve the SUPPLIER and PART tables.