题目

                        Expo 2010 Shanghai China Duration: May 1 to Oct 31, 2010    Welcome to the World Expo 2010 Shanghai China. In order to ensure a safe and orderly visit, the organizer of Expo 2010 has formulated (制订) these rules.    Operation Time    The operation time of the Expo Site shall be 9:00~24:00. The opening hours of the pavilions shall be 9:30 ~ 22:30. Visitors shall enter the Expo Site before 21:00 and leave before 24:00 on the day of admission. Those with evening admission tickets may enter the Expo Site between 17:00 and 21:00.    Or已知a∈(3，4，6)，6∈{0，2，7，8)，r∈{1，8，9}，圆方程(x-a)2＋(y-b)2＝r2可表示不同圆的个数为（　） A．36　　   B．10　　    C．30   D．21