What is/are true of the polymerase chain reaction? a. PCR permits scientists to target a single copy of a single gene among millions of other sequences. b. PCR cannot be used to clone DNA of unknown sequence c. PCR allows scientists to study DNA from samples of finite amounts, such as from fossils, or small tissue samples left at a crime scene. d. PCR allows for the mass production of a specific DNA sequence without cloning. e. all of these