⟷AB and ⟷CD, are straight lines. Which one of these lines is a transversal with respect to the other?
A transversal is a line which intersects two (or more) lines in two (or more) distinct points. However, in this case, either of the lines⟷AB and ⟷CD, intersect only in one point. Hence, neither of ⟷AB and ⟷CD, can be a transversal.