Assertion $A$: Growth is faster in suspension culture compared to callus culture.
Reason $R$: Agitating the culture in the medium using a rotary shaker facilitates the mixing of nutrients and gas exchange.

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