1961 AHSME Problems/Problem 4

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Let the set consisting of the squares of the positive integers be called u; thus u is the set 1,4,9... . If a certain operation on one or more members of the set always yields a member of the set, we say that the set is closed under that operation. Then u is closed under:

(A) addition (B) multiplication (C) division (D) extraction of a positive integral root (E) none of these