Assertion $(A)$: The human body is exposed to a large number of infectious agents daily,yet only a few result in disease. Reason $(R)$: The human body possesses an immune system. Which option is correct for Assertion $(A)$ and Reason $(R)$?

• 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.