Assertion $A$: Gene is the unit of inheritance.
Reason $R$: The properties of tissues are due to the constituents of their cells.
Which option is correct for Assertion $A$ and Reason $R$?

• A
  $A$ and $R$ are true and $R$ is the correct explanation of $A$.
• B
  $A$ and $R$ are 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.