An ideal DNA barcode' sequence will choose one): 1. Be the same across a wide range of species
2. Be variable within species but not between species
3. Be variable between species but not within species
4. Be variable both within and between species

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the answer is third world countries, take for instance china, they restricted each family with only one child,

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relative dating involves things like higher layers are most recent than rocks lower in the sequence. this can be extended to the known sequence of fossils. rocks containing rabbit fossils are later than those containing dinosaur fossils.  

absolute dating methods mainly use radioactive elements that have known rates of decay and produce recognizable products. for example potassium 40 decays to produce argon. once a rock has solidified the argon is trapped. by measuring the argon and potassium0 you can get an accurate number for the age.  

some other methods are good for archaeological ages. dendrochronology involves recognizing the sequence of tree rings. the orientation of magnetized particles in baked clay can be used to date a fire.

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