Cellular phones use radio waves to transmit voice communication in the . . . . . . band.

The wavelength $21 \ cm$ emitted by atomic hydrogen in interstellar space belongs to

Match List-$I$ with List-$II$:

List-$I$ (Electromagnetic wave)	List-$II$ (Production)
$A$. Microwave	$I$. Electrons in atoms emit light when they move from a higher energy level to a lower energy level
$B$. Visible light	$II$. Radioactive decay of nucleus
$C$. Gamma rays	$III$. Vibration of atoms and molecules
$D$. Infra-red rays	$IV$. Klystron valve or magnetron valve

Choose the correct answer from the options given below:

Photons of an electromagnetic radiation have an energy of $11 \, keV$ each. To which region of the electromagnetic spectrum does it belong?

What is the correct order of frequency for $\gamma-$ rays $(b)$,$X-$ rays $(a)$,and $UV-$ rays $(c)$?

Pick out the longest wavelength from the following types of radiations.