light travels through a glass of refractive index $\mu =1.5 $ of thickness $4 mm$. What is the time taken for the light to cross it

Question

$(a)\;2 \times 10^{-11}\;seconds \\ (b)\;4 \times 10^{-11}\;seconds \\ (c)\;6 \times 10^{-11}\;seconds \\ (d)\;8 \times 10^{-11}\;seconds $