Consider the following statements about insulin:
$I$. Human insulin is made up of $51$ amino acids arranged in two polypeptide chains.
$II$. The two polypeptide chains are interconnected by two disulphide bridges.
$III$. In mammals,including humans,insulin is synthesized as a pro-hormone,which contains an extra stretch called the $C$-peptide.
$IV$. $C$-peptide is not present in the mature insulin.
Which of the statements given above are correct?

• A
  $I$,$II$ and $III$
• B
  $I$,$III$ and $IV$
• C
  $II$,$III$ and $IV$
• D
  $I$,$II$,$III$ and $IV$