1. Given the algebraic expression $x^{4} y^{2}+x^{2} y^{4}$, when a monomial is added, it results in the square of an integral polynomial. Then the number of monomials that meet the condition is ( ) .
(A) 3
(B) 4
(C) 5
(D) 6
-,1.D.
The qualified monomials can be
$$
-x^{4} y^{2}, -x^{2} y^{4}, \pm 2 x^{3} y^{3}, \frac{1}{4} x^{6}, \frac{1}{4} y^{6}.
$$