Solve
-A Projectile Is Thrown Upward So That Its Distance $t$

Multiple Choice

Solve.
-A projectile is thrown upward so that its distance above the ground after $t$ seconds is $h ( t ) = - 16 t ^ { 2 } + 420 t$ . After how many seconds does it reach its maximum height?

A) $7 \mathrm { sec }$
B) $28 \mathrm { sec }$
C) $21 \mathrm { sec }$
D) $13 \mathrm { sec }$