(单选题)13: 下列式子可出现负值的是
A: Σ(X-<SPAN style="TEXT-DECORATION: overline">X</SPAN>)2
B: Σ(Y-<SPAN style="TEXT-DECORATION: overline">Y</SPAN>)2
C: Σ(X-<SPAN style="TEXT-DECORATION: overline">X</SPAN>)(Y-<SPAN style="TEXT-DECORATION: overline">Y</SPAN>)
D: ΣX2-(ΣX)2/n
E: ΣY2-(ΣY)2/n
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