Cosmic rays bombard the `"CO"_2` in the atmosphere and, by nuclear reaction, cause the formation of the radioactive carbon isotope C-14. This isotope has a half-life of 5730 years. It mixes into the atmosphere uniformly and is taken up in plants as they grow. After a plant dies, the C-14 decays over the ensuing years. How old is a piece of wood that has a C-14 content which is only 9 percent as large as the average C-14 content of new-grown wood?