Nna is trying to determine if this quadrilateral is an isosceles trapezoid. first, she uses the slope formula and finds that ab || dc. which other step must anna take before she makes a determination? a) use the slope formula to determine if ad || bc. b) use the distance formula to determine if ab = cd. c) use the distance formula to determine if ad = bc. d) use the slope formula to determine if ad ⊥ ab.