Which of the following is not an observation about critical values? choose the correct answer below.

a. a critical value is the number on the borderline separating sample statistics that are likely to occur from those that are unlikely to occur.
b. a critical value is the area in the right-tail region of the standard normal curve.
c. the number z subscript alpha divided by 2 is a critical value that corresponds to an area of 1minusalpha divided by 2 to its left.
d. the number z subscript alpha divided by 2 is a critical value that is a z score with the property that it separates an area of alpha divided by 2 in the right tail of the standard normal distribution.