What purpose would a scientists need to extract dna?

if someone were to commit a crime and there were 3 leads and one piece of evidence, the 3 people would have to be extracted of some dna and so would the piece of evidence, and the person that matches the dna on the evidence would be tied to the crime

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i believe it's c. the trait for large beaks will be passed on to future generations through natural selection.

since eventually smaller beaks would have to find another source of food or even eventually just die out from the lack of food.