Practice GMAT Data Sufficiency Question
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X, Y, and Z are three points in space; is Y the midpoint of XZ?
 ZY and YX have the same length
 XZ is the diameter of a circle with center Y
Correct Answer: B

Evaluate Statement (1) alone.
 It is possible that XZ is a straight line with Y as the midpoint, making ZY=YX.
 However, just because ZY = YX does not mean Y must always be the midpoint; XYZ could be an equilateral triangle.
 Statement (1) alone is NOT SUFFICIENT.

Evaluate Statement (2) alone.
 By definition, the center of a circle is the midpoint of a diameter. Consequently, XZ runs through point Y and XY = YZ since both are radii and all radii must be the same length.
 Statement (2) alone is SUFFICIENT.
 Since Statement (1) alone is NOT SUFFICIENT but Statement (2) alone is SUFFICIENT, answer B is correct.
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