1. The curve $(x+2 y+a)\left(x^{2}-y^{2}\right)=0$ represents three straight lines intersecting at one point in the plane if and only if ( ).
(A) $a=0$
(B) $a=1$
(C) $a=-1$
(D) $a \in \mathbf{R}$
-,1. A.
Since the lines $y=x$ and $y=-x$ intersect at the origin, the curve will be three lines intersecting at one point on the plane if and only if the line $x+2y+a=0$ passes through the origin, i.e., $a=0$.