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For a standard normal distribution, which of the following expressions must always be equal to 1?
A. P (z less-than-or-equal-to a) - P (negative a less-than-or-equal-to z less-than-or-equal-to z) - P (z greater-than-or-equal-to a)
B. P (z less-than-or-equal-to a) - P (negative a less-than-or-equal-to z less-than-or-equal-to z) + P (z greater-than-or-equal-to a)
C. P (z less-than-or-equal-to a) + P (negative a less-than-or-equal-to z less-than-or-equal-to z) minus P (z greater-than-or-equal-to a)
D. P (z less-than-or-equal-to a) + P (negative a less-than-or-equal-to z less-than-or-equal-to z) + P (z greater-than-or-equal-to a)