A double convex lens of the objective is changed to plano-convex. The objective is made of a plastic material with refractive index 1.3. Then,

• A

  the numerical aperture of the double convex lens is 1.3 .
• Correct answer
  B

  the numerical aperture of the plano-convex lens is 0.39 .
• C

  the diameter to focal length ratio of the plano-convex lens is 1.2 .
• D

  the critical angle of the double convex lens is $60^{\circ}$.

Hence, the correct answer is option: 'B'.