Hirudin is

• A
  $A$ protein produced by $Hordeum$ $vulgare$,which is rich in lysine.
• B
  $A$ toxic molecule isolated from $Gossypium$ $hirsutum$,which reduces human fertility.
• C
  $A$ protein produced from transgenic $Brassica$ $napus$,which prevents blood clotting.
• D
  An antibiotic produced by a genetically engineered bacterium,$Escherichia$ $coli$.