"a parallelogram is a rectangle if and only if it has a right angle."

if this statement is true, then which of the following statements must also be true?

a. a parallelogram is not a rectangle if and only if it does not have a right angle. b. a parallelogram is a rectangle if and only if it does not have a right angle.
c. a parallelogram is not a rectangle if and only if it has a right angle.
d. a parallelogram has a right angle if and only if it is not a rectangle.