For a standard normal distribution, which of the following expressions must always be equal to 1?   A) P(z≤-a)-P(-a≤z≤a)+P(z≥a) B) P(z≤-a)+P(-a≤z≤a)-P(z≥a) C) P(z≤-a)+P(-a≤z≤a)+P(z≥a) D) P(z≤-a)-P(-a≤z≤a)+P(z≥a) Answer: - C) P(z≤-a)+P(-a≤z≤a)+P(z≥a)   Explanation: For a standard normal distribution, the expression must always equal 1. This is because P(z≤-a)+P(-a≤z≤a)+P(z≥a) represents all the possible values in the total area of any given curve. The entire area of every given curve must always match the standard normal distribu... More