单选题：“a=3”是直线ax+2y+2a=0和直线3x+(a-1)Y-a+7=0平行的（　　）．

题目内容：

“a=3”是直线ax+2y+2a=0和直线3x+(a-1)Y-a+7=0平行的（　　）． A．充分而不必要条件
B．必要而不充分条件
C．充要条件
D．既不充分又不必要条件

参考答案：

答案解析：

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