Poling process is used for

Trees and shrubs have . . . . . . in their bark for gaseous exchange.

What is the process of transferring cell cultures from an old medium to a new medium called?

$A$ ray of light refracts from medium $1$ into a thin layer of medium $2$,crosses the layer,and is incident at the critical angle on the interface between medium $2$ and $3$ as shown in the figure. If the angle of incidence of the ray is $\theta$,the value of $\theta$ is

For all integers $n \geq 1$, which of the following is divisible by $9$?

In the arrangement shown in the figure,the ends $P$ and $Q$ of an unstretchable string move downwards with a uniform speed $U$. Pulleys $A$ and $B$ are fixed. The mass $M$ moves upwards with a speed: