What type of information do scientist use to determine the approximate age of the earth?

water
layers of rock
fossils
atmospheric pressure

The correct answer is layers of rock.

The age of earth is determined by the layers of rock. By chemical nature rock minerals contain certain radioactive isotopes which can be used to determine the age of the earth. By measuring the concentration of stable end product and knowing the concept of initial concentration and half life,  the age of rocks can be determined. This procedure is used to determine the age of earth.The age of oldest terrestrial rock gives the approximate age of earth.

"1. it should be, it has been for about 20 years, and it's really not much different in the end than breeding animals to have particular traits. there are always concerns about genetically engineered animals/plants affecting the environment in unexpected negative ways, but i can't think of an example where the fears didn't turn out to be baseless.  

first, as you point out, it would be a great advance in pharmacology, to be able to tailor drugs and their doses to me. the pros involve better medical treatment. the cons involve privacy issues like insurance companies denying coverage to people with genetic risk factors. looking at dna markers wouldn't affect the frequency of genetic diseases, but it would allow detection of them earlier. and ok, actually i suppose that if people used this information to avoid having children with genetic diseases, some might decrease in frequency."