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 synthesised 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 $III$ only
• B
  $I, II$ and $IV$ only
• C
  $I, III$ and $IV$ only
• D
  $I, II, III$ and $IV$