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Hope this helps!
we are given the average monthly rainfall (in millimeters) model r(m) for the years 1900-2009 as a function of the month number m.
as we can see m is taken for the number of months for years 1900-2009.
we know that number of months can't be a negative number or decimal number or a fraction.
so, the number of months could be only positive whole numbers.
therefore, whole numbers is more appropriate for the domain of r.