Answer
The correct answer is: $A. 4.5~s$
Work Step by Step
We can find the time it takes the rock to fall $100~m$:
$y = \frac{1}{2}at^2$
$t = \sqrt{\frac{2y}{a}}$
$t = \sqrt{\frac{(2)(100~m)}{10~m/s^2}}$
$t = 4.5~s$
The correct answer is: $A. 4.5~s$
