编写一个简单的统计购书应付款的应用程序。该程序完成在组合框中选择要购买书籍的类别，然后再从列表框中选择要购买的书籍，并统计出应付的钱数。-Write a simple application that counts the book payables. The program is complete in the combo box to the category to buy books, and then the list box to buy the books, and stati