题目内容：
You are developing a page named Products.aspx in a Web application that contains three DropDownList controls that are dynamically loaded from a SQL Server 2005 database file. The DropDownList controls represent a vendor list, a certification list, and an exam list. Certkiller .com customers use the Products.aspx page to select exams related to particular certifications on offer from a particular vendor. A Go button initiates the selection. Each DropDownList control has an associated RequiredFieldValidator control.Whenever the customer selects a vendor, and the vendor offers certifications, then the customer must also select a certification if the. If the vendor only offers exams and not certifications, the certification list should remain hidden. Whenever the customer selects a certification, the customer must also select an exam. Validation error messages should only be displayed when the Go button is clicked. You need to set properties on the vendor DropDownList control.What should you do?（）
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