If two parallel lines are cut by a transversal, which pair of angles are equal?

• A. corresponding angles
• B. vertical angles
• C. adjacent angles
• D. supplementary angles

Answer: (A)

When a transversal crosses two parallel lines, angles that are in the same relative position at each intersection match in measure. Imagine the transversal cutting the first line and forming four angles; at the second intersection, the angle in the same corner (for example, the upper-left at both points) has the same size. This is called corresponding angles, and it happens specifically because the lines are parallel.

Vertical angles are equal, but they occur across the two lines’ crossing at a single point, not across the two different intersections. Adjacent angles share a side and form a straight line, so they sum to 180 degrees rather than being equal. Supplementary angles also sum to 180, not necessarily equal. So the equal pair described here is the corresponding angles.