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.