Palaeontologists unearthed a human skull during excavation. $A$ small fragment of the scalp tissue was still attached to it. Only a little $DNA$ could be extracted from it. If the genes of the ancient man need to be analysed,the best way of getting a sufficient amount of $DNA$ from this extract is:

• A
  By hybridizing the $DNA$ with a $DNA$ probe
• B
  By subjecting the $DNA$ to polymerase chain reaction
• C
  By subjecting the $DNA$ to gel electrophoresis
• D
  By treating the $DNA$ with restriction endonucleases