1. If real numbers $x, y$ satisfy $\left(\sqrt{x^{2}+1}+x\right)$
- $\left(\sqrt{y^{2}+1}+y\right)=1$, then the value of $x+y$ is ( ).
(A) -1
(B) 0
(C) 1
(D) cannot be determined
-1 (B).
Hint: Given that both sides of the equation are multiplied by $\sqrt{x^{2}+1}-x$,
$$
\sqrt{y^{2}+1}-y
$$