拿到0个红球的概率 = C(4,3)/C(9,3) = 1/21
拿到1个红球的概率 = C(5,1)C(4,2)/C(9,3) = 5/14
拿到2个红球的概率 = C(5,2)C(4,1)/C(9,3) = 10/21
拿到3个红球的概率 = C(5,3)/C(9,3) = 5/42
拿到2个红球的概率最大