If ${\varepsilon _0}$ and ${\mu _0}$ are respectively the electric permittivity and the magnetic permeability of free space,and ${\varepsilon}$ and ${\mu}$ are the corresponding quantities in a medium,the refractive index of the medium is:

• A
  $\sqrt {\frac{{\mu \varepsilon }}{{{\mu _0}{\varepsilon _0}}}} $
• B
  $\frac{{\mu \varepsilon }}{{{\mu _0}{\varepsilon _0}}}$
• C
  $\sqrt {\frac{{{\mu _0}{\varepsilon _0}}}{{\mu \varepsilon }}} $
• D
  $\sqrt {\frac{{\mu {\mu _0}}}{{\varepsilon \,{\varepsilon _0}}}} $