**A**

**Answer: D. P(z≤-a)+P(-a≤z≤a)+P(Z≥a)**

Explanation: or 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. In agreement with the formula for expressing standard normal distribution, the total area of any given curve must always equal 1.

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