The operation of a nuclear reactor is said to be critical if the multiplication factor $(K)$ has a value of:

$A$ nuclear reactor provides a power of $300 \, MW$. If the fission of one uranium nucleus ${U^{235}}$ releases $170 \, MeV$ of energy,how many nuclei undergo fission in $1 \, h$?

Match the following items in Column-$A$ with their corresponding principles in Column-$B$:

Column-$A$	Column-$B$
$A$. Rocket propulsion	$P$. Bernoulli's principle in fluid dynamics
$B$. Aeroplane	$Q$. Total internal reflection of light
$C$. Optical fibres	$R$. Newton's laws of motion
$D$. Fusion test reactor	$S$. Magnetic confinement of plasma
	$T$. Photoelectric effect

How much $MeV$ energy is emitted by the fission of one nucleus of uranium-$235$ (in $MeV$)?

$A$ ${ }_{92} U^{235}$ nucleus absorbs a neutron and disintegrates into ${ }_{54} X e^{139}$,${ }_{38} S r^{94}$,and $X$. What will be the product $X$?

Light energy emitted by stars is due to