Given below are two statements: One is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A$: Thin layer chromatography is an adsorption chromatography.
Reason $R$: $A$ thin layer of silica gel is spread over a glass plate of suitable size in thin layer chromatography which acts as an adsorbent.
In the light of the above statements,choose the correct answer from the options given below.

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