If two parallel lines are intersected by a transversal, then which of the options below is necessarily true?
AThe sum of the measures of a pair of alternate interior angles is 180°
BThe sum of the measures of a pair of corresponding angles is 180°
CThe sum of the measures of a pair of interior angles on the same side of the transversal is 180°
DThe sum of the measures of a pair of vertically opposite angles is 180°