Which graph best shows the velocity-time graph for an object launched vertically into the air when air resistance is given by $|D| = bv$? The dashed line shows the velocity graph if there were no air resistance.

Due to the presence of air resistance, if a body dropped from a height of $20 \ m$ reaches the ground with a speed of $18 \ m/s$, then the time taken by the body to reach the ground is nearly (in $s$)

Assertion $(A)$: The displacement of a vertically projected body during the last second of its upward motion is $\frac{g}{2}$.
Reason $(R)$: For a vertically projected body,the acceleration decreases gradually and becomes $\frac{g}{2}$ during the last second of upward motion.

$A$ stone is dropped from the top of a tall building and after $2 \,s$ another stone is thrown vertically downwards with a velocity $5 \,m/s$ from the same point. The distance from the top of the building at which the second stone overtakes the first is . . . . . . $\left(g=10 \,m/s^2\right)$ (in $\,m$)

$A$ boy is standing on top of a tower of height $85 \, m$ and throws a ball in the vertically upward direction with a certain speed. If $5.25 \, s$ later he hears the ball hitting the ground,then the speed with which the boy threw the ball is .......... $m/s$ (take $g = 10 \, m/s^2$ and speed of sound in air $= 340 \, m/s$).

$A$ body at rest falls through a height $h$ with velocity $V$. If it has to fall down further for its velocity to become $3V$,the distance travelled in that interval is: (in $h$)