Question

Which biconditional is NOT a good definition? A whole number is even if and only if it is divisible by 2. Two line segments are congruent if and only if they have same length. Three points are collinear if and only if they lie on the same line. Two lines are parallel if and only if they do not intersect..

Answer 1

Go through each biconditional and read it backwards to see if there is a case that is not true.
For example, start with A. A whole number is even if and only if it is divisible by 2.
Statement:
A whole number is even if it is divisible by 2.
Now do it backwards.
If a number is divisible by 2, it is even.
Is the backwards reading also true?

Answer 2

Do the same for all choices.