What is the primary objective of crop rotation?

$A$ wire carrying current $I$ is shaped as shown in the figure. Section $AB$ is a quarter circle of radius $r$. The magnetic field at point $C$ is directed:

If $L: ax + by + c = 0$ is a variable straight line,where $a, b$ and $c$ are the second,fourth and seventh terms of an $AP$ respectively,then $L$ passes through the fixed point -

Which of the following prevents the collapsing of the trachea?

$A$ massless string is wrapped around a disc of mass $M$ and radius $R$. Another end is tied to a mass $m$ which is initially at height $h$ from ground level as shown in the figure. If the mass is released,then its velocity while touching the ground level will be

In which plant group are the sporophylls arranged spirally along an axis to form distinct compact structures called strobili or cones?