A ball is dropped from a height of $20\text{m}$. At the same instant, another ball is thrown upward from the ground with a speed of $20\text{m/s}$ along the same line. After what time will the two balls meet?

The correct option is D $1\text{s}$
We will solve this by using the concept of relative motion.
Relative initial speed, $v=20-0=20\text{m/s}$
Relative acceleration, $a=-10-(-10)=0$
Relative initial separation, $d=20\text{m}$

So, the relative motion is uniform motion.
$\therefore t=\frac{d}{v}=\frac{20}{20}=1\text{s}$