$19 \cdot 13$ （1）From $x^{2}-y^{2}=0$, we can get $\left\{\begin{array}{l}x+y=0, \\ x-y=0 \text {. }\end{array}\right.$
(2) If a line has two points that are equidistant from another line, then the two lines are parallel.
(3) If two triangles have two corresponding sides equal, and their areas are also equal, then they are congruent.
（4）In a heptagon, at least four of the interior angles are obtuse.
The number of correct propositions among the above is
(A) 1 .
(B) 2 .
(C) 3 .
(D) 4 .
(3rd "Jinyun Cup" Junior High School Mathematics Invitational Competition, 1986)
[Solution] (1) From $x^{2}-y^{2}=0$ we get $x+y=0$ or $x-y=0$, so the original statement is not true.
(2) As shown in the right figure (1), the original statement is not true.
(3) As shown in figure (2), $AD$ is the median to the base $BC$ of $\triangle ABC$, $\triangle ABD$ and $\triangle ADC$ satisfy the given conditions, but they are not congruent. Therefore, the original statement is not true.
(4) If a convex heptagon has at most three obtuse angles, then it has at least four acute angles, which means its exterior angles have at least four obtuse angles, and their sum exceeds $360^{\circ}$, leading to a contradiction. Therefore, the original statement is correct. Hence, the answer is $(A)$.