Explain the difference between $nm$, $mN$, and $Nm$.

Match List-$I$ with List-$II$ and select the correct answer by using the codes given below the lists.

List-$I$	List-$II$
$(A)$ Distance between earth and stars	$(1)$ Microns
$(B)$ Inter-atomic distance in a solid	$(2)$ $\mathring{A}$s
$(C)$ Size of the nucleus	$(3)$ Light years
$(D)$ Wavelength of infrared laser	$(4)$ Fermi

What is heat? Write $SI$ and $CGS$ unit of heat and give their relation.

Express $1$ atm pressure into $N/m^{2}$ and bar.

The new $SI$ unit of mass $1 \, kg$ is defined in terms of the difference in the masses of two ${}^{133}Cs_{55}$ atoms. One of these atoms is in its ground state and the other is in an excited state that has a frequency of excitation close to $9.2 \times 10^9 \, Hz$. The number of atoms required to get $1 \, kg$ of mass this way is of the order of (Planck's constant = $6.63 \times 10^{-34} \, J \cdot s$; mass of proton = $1.67 \times 10^{-27} \, kg$; Avogadro number = $6.02 \times 10^{23} \, \text{particles}$; speed of light = $3 \times 10^8 \, m/s$).

What is $1\,AU$? It represents which physical quantity?