The height of a projectile launched from the ground is modeled by the equation $h(t)=-16t^2+80t$, where $h(t)$ is the height in feet and $t$ is the time in seconds after launch. When the projectile hits the ground, $h(t)=0$. Solving the equation $-16t^2+80t=0$ yields two solutions for $t$: $t=0$ and $t=5$. Which of the following statements best explains why $t=5$ is the only valid time for the projectile to hit the ground after launch?