Assertion $(A)$: Graphite is used as a dry lubricant in machines which run at high temperatures.
Reason $(R)$: The layers of graphite slip one over the other when pressure is applied.
The correct option among the following is:

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