When the object is self- luminous, the resolving power of a microscope is given by the expression :

• A

  $\frac{2\mu \sin \theta }{\lambda }$

• B

  $\frac{\mu \sin \theta }{\lambda }$

• C

  $\frac{2\mu \cos \theta }{\lambda }$

• D

  $\frac{2\mu }{\lambda }$