Given below are two statements: one is labelled as Assertion $(A)$ and the other is labelled as Reason $(R)$.
Assertion $(A)$: Refractive index of glass is higher than that of air.
Reason $(R)$: Optical density of a medium is directly proportionate to its mass density which results in a proportionate refractive index.
In the light of the above statements,choose the most appropriate answer from the options given below.

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