Let A.B.C be any real number which is not completely equal, if x = a-bc, y = b-ac, z = c-ab, Z, then x, y and Z are all less than 0 and no more than 0, then Connect. C at least one ＜ 0, D at least one ＞ 0

X + y + Z = a + B + C - (AB + BC + AC) = (a-b) / 2 + (B-C) / 2 + (A-C) / 2 ≥ 0, if and only if a = b = C, x + y + Z = 0, then there must be one greater than 0, so D is chosen

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