Which of the following expressions is a polynomial? State the reason. If an expression is a polynomial,state whether it is a polynomial in one variable or not: $x^{2}+y^{2}+z^{2}+2xy+2yz+2zx$

(N/A) The expression $x^{2}+y^{2}+z^{2}+2xy+2yz+2zx$ is a polynomial because it consists of variables with non-negative integer exponents and coefficients that are real numbers. Since it contains three distinct variables $(x, y, z)$,it is a polynomial in three variables,not a polynomial in one variable.