Which biconditional is NOT a good definition?

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.
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

Question

Which biconditional is NOT a good definition?
 
   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. 
 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

anonymous · Answer

Identify a counterexample for the following conditional: 

If you live in Springfield, then you live in Illinois.
 
   Billy Jones lives in Chicago, Illinois. 
 Sara Lucas lives in Springfield. 
 Jonah Lincoln lives in Springfield, Illinois. 
 Erin Naismith lives in Springfield, Massachusetts.