Below are two statements. One is labeled as Assertion $(A)$ and the other as Reason $(R).$
Assertion $(A):$ Gypsum is used to make boards for fireproof walls.
Reason $(R):$ Gypsum is unstable at high temperatures.
In the context of the above statements,choose the correct answer from the options given below:

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