Which biconditional is not a good definition?

Question

A whole number is odd if and only if the number is not divisible by 2.

• An angle is straight if and only if its measure is 180°
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A whole number is even if and only if it is divisible by 2.

A ray is a bisector of an angle if and only if it splits the angle into two angles.

Answer 1

The biconditional "A ray is a bisector of an angle if and only if it splits the angle into two angles" is not a good definition.