Which biconditional is not a good definition? two line segments are congruent if and only if they are of same length. an angle is straight if and only if its measure is 180. two figures are congruent if and only if their areas are equal. a whole number is odd if and only if the number is not divisible by 2. its not b

You are looking for something that is not so.
D, for example, is true. That's the definition of an odd number.

B, as you found out, is also true.

A is the definition of a straight angle.

The answer is C. C is not true. You can draw a triangle. Then draw a line parallel to the base going through the point of the triangle not on the base. If you slide that third point anywhere along the parallel line, the areas are the same for the entire family of triangles that you draw. 

C<<<<< answer.