public String toString(double weight, double height){ double b = weight/(height*height); if(b < 20) return "偏瘦"; else if(b < 25) return "正常"; else if(b < 30) return "偏胖"; else if(b < 35) return "肥胖"; else if(b < 40) return "重度肥胖"; else return "极度肥胖";}如果采用等价类测试方法，最少要为这个程序设计______ 组测试数据？