1. After adding a monomial to the algebraic expression $x^{4}+x^{2}$, the result is a perfect square of an algebraic expression. Then, the number of such monomials is ( ).
(A) 6
(B) 5
(C) 4
(D) no more than 3
-1. A.
The monomials that meet the conditions are only
$$
-x^{4},-x^{2}, \frac{1}{4}, \pm 2 x^{3}, \frac{1}{4} x^{6}
$$