1. The curve $(x+2 y+a)\left(x^{2}-y^{2}\right)=0$ represents three straight lines intersecting at one point on the plane if and only if
A. $a=0$
B. $a=1$
C. $a=-1$
D. $a \in \mathbf{R}$
Obviously, the three lines are $x+2y+a=0$, $x+y=0$, and $x-y=0$. Since $x+y=0$ and $x-y=0$ intersect at point $(0,0)$, substituting into $x+2y+a=0 \Rightarrow a=0$, so the answer is $A$.

