Which ones are polynomials, and which are not?

x^-5-5x^-4+4x^-3-3x^-2+2x^-1-1

3x^2-5x^4+2x-12

x^5-5x^4+4x^3-3x^2+2x-1

x^3-7x^2+9x-5x^4-20

4/x^4+3/x^3-2/x^2-1

\sqrt[4]{x}-\sqrt[3]{x}+4\sqrt{x}-8x+16