Why Is PCR or Polymerase Chain Reaction Such a Big

Why is PCR or polymerase chain reaction such a big deal that Dr. Mullis shared a Nobel Prize in chemistry for developing it?

A) Previously, no one understood that it is the polymerase in a cell which copies DNA, a hugely important task.
B) Previously, if one wanted to copy a gene, it was necessary to copy a whole genome, not just the part you wanted.
C) Because it allowed for the insertion of genes from one species into another.
D) It made it possible to quickly and easily make enough copies of a desired gene to work with for genetic engineering or DNA analysis.
E) It made it possible to quickly and easily transform a cell with new DNA.