Assertion (A) When an ideal gas is compressed adiabatically, its temperature and the average kinetic energy of the gas molecules increase. Reason (R) The kinetic energy increases because of collisions of molecules with moving parts of wall only.

• Correct answer
  A

  $\mathrm{A}$ and $\mathrm{R}$ and true and $\mathrm{R}$ is correct explanation of A.
• B

  $\mathrm{A}$ and $\mathrm{R}$ are true but $\mathrm{R}$ is not correct explanation of $A$.
• C

  $A$ is true and $R$ is false.
• D

  $\mathrm{A}$ is false and $\mathrm{R}$ is true.

Hence, the correct answer is option: 'A'.