$Assertion (A)$: White tin is an example of a tetragonal system. $Reasoning (R)$: For a tetragonal system, $a=b=c$ and $\alpha=\beta=\gamma \neq 90^{\circ}$. The correct answer is

• A
  $A$ and $R$ are true and $R$ is the correct explanation of $A$.
• B
  Both $A$ and $R$ are true and $R$ is not the correct explanation of $A$.
• C
  $A$ is true but $R$ is not true.
• D
  $A$ is not true but $R$ is true.