Which is the best method to obtain virus-free plants through tissue culture?

If $(a+bx)^{-3} = \frac{1}{27} + \frac{1}{3}x + \dots$,then the ordered pair $(a, b)$ equals to

If $z=\log (\tan x+\tan y)$,then $(\sin 2x) \frac{\partial z}{\partial x}+(\sin 2y) \frac{\partial z}{\partial y}$ is equal to

The function $f: R \rightarrow R$ defined by $f(x)=\frac{x}{\sqrt{1+x^2}}$ is ........

$A$ child is standing at one end of a long trolley moving with a speed $v$ on a smooth horizontal floor. If the child starts running towards the other end of the trolley with a speed $u$,the centre of mass of the system (trolley + child) will move with a speed:

The solubility of $N_2$ in water at $300 \ K$ and $500 \ torr$ partial pressure is $0.01 \ g \ L^{-1}$. The solubility (in $g \ L^{-1}$) at $750 \ torr$ partial pressure is