Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A$: An electron microscope can achieve better resolving power than an optical microscope.
Reason $R$: The de Broglie wavelength of the electrons emitted from an electron gun is much less than the wavelength of visible light.
In the light of the above statements, choose the correct answer from the options given below:

• A
  $A$ is true but $R$ is false.
• B
  Both $A$ and $R$ are true and $R$ is the correct explanation of $A$.
• C
  Both $A$ and $R$ are true but $R$ is $NOT$ the correct explanation of $A$.
• D
  $A$ is false but $R$ is true.