3. Haploid reproductive cells.

A haploid reproductive cell, either egg or sperm. Possessing a single set of chromosomes. Human gametes (eggs and sperm) are haploid cells because they only have a signle set of chromosomes. All other cells in the human body are diploid, meaning they possess two sets of chromosomes.A cell whose nucleus unites with that of a cell of the opposite sex to form a new organism. A reproductive cell contains only a single (haploid) set of chromosomes. Animal egg and sperm cells, the nuclei in grains of pollen, and egg cells in plant ovules are all reproductive cells. Also called gamete, sex cell

Explanation:

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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