Decide whether each of the given sets is a group with respect to the indicated operation. If it is not a group, state a condition in the definition of group that fails to hold.
(a) The set Z+ of all positive integers with operation multiplication.
(b) For a fixed integer n, the set of all complex numbers x such that xn = 1 (That is, the set of all nth roots of 1), with operation multiplication.
(c) The set Q' of all irrational numbers with operation addition.
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