Choose the correct answer below.
a. a critical value is the area in the right-tail region of the standard normal curve.
b. the number z_α/2 is a critical value that corresponds to an area of (1-α)/2 to its left.
c. the number z_α/2 is a critical value that is a z score with the property that it separates an area of α/2 in the right tail of the standard normal distribution.
d. a critical value is the number on the borderline separating sample statistics that are likely to occur from those that are unlikely to occur.