A polymerase chain reaction (PCR) for amplification of DNA fragments involves multiple cycles of thermal denaturation, annealing and extension steps in each cycle. Because of the heat denaturation step, a thermostable polymerase is required for performing PCR. An experimenter had all the resources except the thermostable DNA polymerase to perform a PCR. The experimenter can use ______ to substitute for thermostable DNA polymerase.

• A

  Escherichia coli DNA polymerase added before denaturation step of every PCR cycle
• B

  Escherichia coli DNA polymerase, added after extension step of every PCR cycle
• C

  excess amount of Escherichia coli DNA polymerase added once at the beginning of the reaction
• Correct answer
  D

  Escherichia coli DNA polymerase, added after annealing step of every PCR cycle

Hence, the correct answer is option: 'D'.