Answer
This is not a function.
Work Step by Step
A function is a correspondence between a first set, called the domain,
and a second set, called the range,
such that EACH member of the domain corresponds to EXACTLY ONE member of the range.
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We ask:
1. Does EACH member of the band play an instrument?
Not necessarily. A singer, for example, needs not play an instrument.
2. Can a member of the band know how to play more than one instrument?
Yes, so the EXACTLY ONE clause is also violated.
This is not a function.
