The function f is defined for all positive integers n by the following rule: f(n) is the number of positive integers each of which is less than n and has no positive factor in common with n other than 1. If p is any prime number then f(p) =
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A p - 1
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B p - 2
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C$${(p+1)}\over{2}$$
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D$${(p-1)}\over{2}$$
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E 2