12. is a form of artificially acquired immunity that occurs when the
majority of a population, but not all, has been given a vaccine and becomes
resistant to infection. It only works if the majority is vaccinated.

c.) polymerase chain reaction

- "unlike the original dna fingerprinting method, dna profiling does not use restriction enzymes to cut the dna. instead it uses the polymerase chain reaction (pcr)? to produce many copies of specific str sequences. pcr is an automated procedure that generates lots of copies of a specific sequence of dna."

for a different answer set the answer would be because the oxygen in the water had depleted (apex)