I'm thinking the answer is a.
the photograph shows a laboratory investigator examining samples of dna in a procedure called pcr, or polymerase chain reaction. using pcr, the investigator can create thousands or even millions of copies of a fragment of dna for closer investigation. in which of these situations would pcr be most useful?
a. when only a small amount of dna is available, such as at a crime scene
b. when the dna of identical twins is being mapped
c. when the genome of a population is being analyzed for preventable diseases
d. when the blood type of a dna donor is known