Assertion $A$: Continued inbreeding is not desirable.
Reason $R$: The fertility of offspring decreases due to the accumulation of harmful recessive genes.

• 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
  $A$ and $R$ are both false.