Assertion : Standard optical diffraction gratings cannot be used for discriminating between different $\mathrm{X}$-ray wavelengths. Reason : The grating spacing is not of the order of $\mathrm{X}$-ray wavelengths.

• Correct answer
  A

  If both assertion and reason are true and reason is the correct explanation of the assertion
• B

  If both assertion and reason are true but reason is not the correct explanation of the assertion
• C

  If assertion is true, but reason is false
• D

  Both assertion and reason are false statements.

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