For the following Assertion and Reason,the correct option is
Assertion $(A) :$ When $Cu^{2+}$ and sulphide ions are mixed,they react together extremely quickly to give a solid.
Reason $(R) :$ The equilibrium constant of $Cu^{2+}_{(aq)} + S^{2-}_{(aq)} \rightleftharpoons CuS_{(s)}$ is high because the solubility product is low.

• A
  Both $(A)$ and $(R)$ are true and $(R)$ is the correct explanation for $(A)$
• B
  Both $(A)$ and $(R)$ are false
• C
  $(A)$ is false and $(R)$ is true
• D
  Both $(A)$ and $(R)$ are true but $(R)$ is not the correct explanation for $(A)$