tree whose radioactivity is 1/4 of that of a comparable but relatively young tree. 10. Radiocarbon dating is used to (a) treat a disease by means of radiation; (b) study and apply chemical techniques to the purification of radioactive materials; (c) estimate the age of wooden archaeological objects; (d) all of these. (c) 1 1. The age of the earth is estimated by (a) Uranium dating; (b) radiocarbon dating: (c) atomic clocks; (d) biological clocks. (a) 12. Radiocarbon in nature is produced as a result of the action of (a) cosmic rays on earth's atmospheric nitrogen; (b) ultraviolet rays on ordinary carbon; (c) X-rays on ordinary carbon; (d) infrared rays on ordinary carbon. (a) 13. The radio isotopes are used (a) to study, diagnose and treat certain diseases; (b) in various kinds of environmental studies; (c) to measure the thickness of materials by measuring the radiation passing through them; (d) all the above. (d) 14. Gamma radiations are used for (a) cancer therapy; (b) sterilising foodstuff; (c) controlling pests; (d) all of these. (d) 15. The ozone layer absorbs (a) a-rays; (b) cosmic rays; (c) Infrared rays; (d) ultraviolet rays coming from the sun. (d) e )