What are molecular clocks used for A- to cure hereditary disease
B- to classify
C- to locate fossils under water
D- to compare homologous structures

B-to classify

when atoms of different types of elements join together, they make molecules called compounds. water consists of compound molecules made up of 2 hydrogen atoms and 1 oxygen atom. this is why it's called h2o. water will always have 2 times the number of hydrogen atoms as oxygen atoms.

looks like the answer is     c     to me!