Assertion $A$: Milk is a useful product used as a valuable food.
Reason $R$: It is the fresh natural secretion of the mammary glands of female mammals for the nourishment of their young ones.
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.