Because Visio does not explicitly show associative entities, it is not clear in Figure 2-21 which entity types are associative. List the associative entities in this figure. Why are there so many associative entities in Figure 2-21?

SALESPERSON DOES BUSINESS IN PK Salesperson ID TERRITORY Serves PK Territory ID Salesperson Name Salesperson Telephone Salesperson Fax Territory Name CUSTOMER PRODUCT LINE PK Product Line ID PK Customer ID Customer Name Product Line Name Customer Address Customer Postal Code Submits ORDER Includes PK Order ID Order Date PRODUCT ORDER LINE PK Product ID Product Description Ordered Quantity Product Finish Product Standard Price VENDOR PRODUCED IN USES PK Vendor ID Vendor Name Goes Into Quantity Vendor Address RAW MATERIAL WORK CENTER SUPPLIES PK Material ID PK Work Center ID Material Name Material Standard Cost Work Center Location Supply Unit Price Unit Of Measure Is Supervised By EMPLOYEE WORKS IN Supervises O PK Employee ID HAS SKILL SKILL He Employee Address Employee Name PK Skill