Which of the following expressions is a polynomial in one variable?

A polynomial in one variable must have only non-negative integer powers of the variable and only one variable. Option C has variable $x$ with powers $2$ and $1$, which are non-negative integers. Option A has $2/x = 2x^{-1}$, Option B has $\sqrt{x} = x^{1/2}$ (not a whole number power), and Option D has two variables ($x$ and $y$).