A Projectile Is Thrown Upward So That Its Distance Above $t$

Multiple Choice

A projectile is thrown upward so that its distance above the ground after $t$ sec is given by $h(t) =-15 t^{2}+600 t$ . After how many seconds does it reach its maximum height?

A) $10 \mathrm{sec}$
B) $20 \mathrm{sec}$
C) $30 \mathrm{sec}$
D) $40 \mathrm{sec}$