Boron can be obtained by various methods but not by :

Which substance is used as a smoke screen in warfare?

If $u=f(r)$,where $r^2=x^2+y^2$,then $\left(\frac{\partial^2 u}{\partial x^2}+\frac{\partial^2 u}{\partial y^2}\right)$ is equal to

$A$ particle carrying a charge equal to $100$ times the charge on an electron is rotating per second in a circular path of radius $0.8 \ m$. The value of the magnetic field produced at the centre will be (where ${\mu _0}$ is the permeability of vacuum).

The angle between the tangents drawn from the point $(1,4)$ to the parabola $y^2=4x$ is

Which part of the body secretes the hormone secretin?