Which of the following bonds is the strongest?

If in a $\triangle ABC$,$r_1 < r_2 < r_3$,then:

The existence of a substance in more than one solid modification is known as:

The figure shows the variation of the gravitational acceleration $a_g$ of four planets with the radial distance $r$ from the centre of the planet for $r \geq$ radius of the planet. Plots $1$ and $2$ coincide for $r \geq R_2$,and plots $3$ and $4$ coincide for $r \geq R_4$. The sequence of the planets in the descending order of their densities is:

If $x > 0$ and $x^y = e^{x-y}$,then $\frac{dy}{dx}$ is equal to

The process of nitrogen fixation by $Rhizobium$ is stimulated by the presence of which of the following?