none of these
$2$
$120$
$24$
$6$

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$\frac{dv}{dt}=t^{5}e^{-t}$

$dv=t^{5}e^{-t}dt$

$\int_0^{v}dv=\int_0^{\infty}t^{5}e^{-t}dt$

$v=\Gamma(6)$

$v=5!=120$