单项选择题
A.6 B.8 C.24 D.12
A.(X1+X2+X3)-(X4+X5+X6) B.X1-X4 C.X3-X6 D.(X3-X1)-(X6-X4)
A.13 B.17 C.22 D.33
A.550 B.600 C.650 D.700
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