Practice GMAT Data Sufficiency Question

X, Y, and Z are three points in space; is Y the midpoint of XZ?

A)	Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient.
B)	Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient.
C)	BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient.
D)	EACH statement ALONE is sufficient.
E)	Statement (1) and (2) TOGETHER are NOT sufficient to answer the question asked, and additional data are needed.

Correct Answer: B

Evaluate Statement (1) alone.
1. It is possible that XZ is a straight line with Y as the midpoint, making ZY=YX.
2. However, just because ZY = YX does not mean Y must always be the midpoint; XYZ could be an equilateral triangle.
3. Statement (1) alone is NOT SUFFICIENT.
Evaluate Statement (2) alone.
1. 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.
2. Statement (2) alone is SUFFICIENT.
Since Statement (1) alone is NOT SUFFICIENT but Statement (2) alone is SUFFICIENT, answer B is correct.