The figure below shows a quadrilateral abcd with diagonal bd bisecting angle adc:

a quadrilateral abcd is drawn with bd as a diagonal. bd bisects angle adc and forms two triangles abd and cbd. ab and cb are equal in length. (i attached a picture to the question).

which equation is true?
a) db = 2dc
b) ad = dc
c) ab = db
d) ab = 2ad