Assertion $A$: Mitochondria are the powerhouse of the cell.
Reason $R$: The sites for cellular respiration,specifically the $Krebs$ cycle and oxidative phosphorylation,are located within the mitochondria.
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.