Which of the following statements is correct regarding the pupil of the human eye?
$(I)$. It is the aperture surrounded by the iris.
$(II)$. The diameter of the pupil is regulated by the muscle fibres of the iris.
$(III)$. It is a transparent crystalline structure attached to the ciliary body.
The correct option is:

• A
  Only $(I)$
• B
  Only $(III)$
• C
  $(I)$ and $(II)$
• D
  $(I, II)$ and $(III)$