Answer
It is not reasonable to use this kind of function, because the sales can not go to negative numbers, whereas the function would do so in the long term.
Work Step by Step
The function of $s(t)=at^2+bt+c$, if $a$ is negative, it is a negative parabola. Meaning that, in the long term sales will have negative value according to this function. In the real life, this can not happen, therefore it is not reasonable to use this kind of function.
