Option C:
The last two terms given of the standard polynomial are:
This shows 2 patterns to us:
r 's degree is increasing per term in the given polynomial.
s 's degree is decreasing per term in the given polynomial.
The sum of powers of variates r and s is 2+4 = 3+3 = 6.
And since standard polynomial, the same pattern must be present in all terms of the series.
All options except C are not valid.
Option C has term for which r's degree plus s's degree is 1 + 5 = 6.
And r's degree 1 < s's degree 5.
Thus its a valid term to be putted as first term in standard form of polynomial in consideration.
Thus Option C: is the needed expression.
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