Which of the following represents the complexity of body organization?

Box $A$ contains $2$ black and $3$ red balls, while Box $B$ contains $3$ black and $4$ red balls. Out of these two boxes, one is selected at random; and the probability of choosing Box $A$ is double that of Box $B$. If a red ball is drawn from the selected box, then the probability that it has come from Box $B$ is:

The maximum value of $xy$ subject to $x+y=7$ is

$A$ pendulum suspended from the ceiling of a train oscillates with a time period $2\,s$,when the train is accelerating at $10\,m/s^2$. What will be its time period when the train retards at $10\,m/s^2$?

$A$ student is to answer $10$ out of $13$ questions in an examination such that he must choose at least $4$ from the first $5$ questions. The number of choices available to him is

Arrange the following compounds in the increasing order of their dipole moment:
$(I)$ Toluene
$(II)$ $m$-Dichlorobenzene
$(III)$ $o$-Dichlorobenzene
$(IV)$ $p$-Dichlorobenzene