已知a∈{3，4，6}，b∈{1，2，7，8}，r∈{5，9}，则方程（x-a）2+（y-b）2=r2可表示（ ）个不同的圆．
A．36
B．24
C．12
D．6
对于两个圆来说，只要它们的圆心不同，或半径不同，两个圆就是平面上不同的圆，
集合{3，4，6}中的任意一个数都可以作为圆心的横坐标，集合{1，2，7，8，}中的任意一个数都可以作为圆心的纵坐标，所以组成的圆心总数为3×4=12种，而半径可以从{5，9}中任选一个，有两种方法，所以，方程（x-a）2+（y-b）2=r2可表示12×2=24个不同的圆．
故选B．
| 阅读理解。 | ||||||||||||||||||||
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Celebrations Around the World
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