The greatest evolutionary change enabling land vertebrates to be completely free from water habitats was the development of:

Which organ in an adult human causes the least damage if removed?

Which of the following hormones does not naturally occur in plants?

The mean of four observations is $3$. If the sum of the squares of these observations is $48$,then their standard deviation is

$A$ whistle $S$ of frequency $f$ revolves in a circle of radius $R$ at a constant speed $v$. What is the ratio of the largest and smallest frequency detected by a detector $D$ at rest at a distance $2R$ from the center of the circle as shown in the figure? (Take $c$ as the speed of sound.)

Two persons $A$ and $B$ are located in the $X-Y$ plane at the points $(0,0)$ and $(0,10)$ respectively. (The distances are measured in $MKS$ units). At time $t=0$,they start moving simultaneously with velocities $\overrightarrow{v}_A = 2\hat{j} \text{ m/s}$ and $\overrightarrow{v}_B = 2\hat{i} \text{ m/s}$ respectively. The time after which $A$ and $B$ are at their closest distance is