Which of the following is a plasmid?

$A$ balloon with mass $m$ is descending down with an acceleration $a$ (where $a < g$). How much mass should be removed from it so that it starts moving up with an acceleration $a$?

$\cos 36^{\circ} - \cos 72^{\circ}$ is equal to

$A$ thermodynamic system undergoes a cyclic process $ABCDA$ as shown in the figure. The work done by the system in the cycle is

The decreasing order of reactivity of the compounds given below towards solvolysis under identical conditions is:

The magnetic field $(B)$ inside a long solenoid having $n$ turns per unit length and carrying current $I$ when an iron core is kept in it is ($\mu_0 = $ permeability of vacuum,$\chi = $ magnetic susceptibility).