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This problem trains for: SAT-I, AMC-8, GMAT.

In the figure below (not drawn to scale) the lines L, L' and L'' are parallel to each other and the lines D and D' are perpendicular to each other. Assume the relative positions of the lines in each set are kept fixed. By how many degrees of angle should we rotate the set D and D' in order to form a rectangle?

figure

It is not possible, since only one of the D lines can be perpendicular to one of the L lines at a time.