  The Veneziano amplitude is derived from fixing one degree of freedom of
$PSL(2,\mathbb{R})$ symmetry by the insertion of a mostly BRST exact operator.
Evaluating the five-point function which consists of four open string tachyons
and this gauge fixing operator, we find it equals the Veneziano amplitude up to
a sign factor. The sign factor is interpreted as a signed intersection
number.The result implies that the mostly BRST exact operator, which is
originally used to provide two-point string amplitudes, correctly fixes the
$PSL(2,\mathbb{R})$ gauge symmetry for general amplitudes. We conjecture an
expression for general $n$-point tree amplitudes with an insertion of this
gauge fixing operator.
