Question

when is the polymerase chain reaction (pcr) a useful technique?
when no sample of dna exists
when an abundance of dna sample exists
when the dna evidence has been contaminated
when a limited sample of dna exists
pcr is not a useful technique because it is considered \junk science\ by scientists and juries.

Explanation:

PCR amplifies small amounts of DNA into large quantities, making it useful when only a limited DNA sample is available. It requires at least a small initial DNA sample (so not when no sample exists), is unnecessary for abundant samples, and does not fix contamination. It is a widely accepted scientific technique, not "junk science."

Answer:

when a limited sample of DNA exists