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A. Chlorofluorocarbons

Explanation:

Halogens are a chemical family containing fluorine, chlorine, bromine and iodine; any carbon compound containing them is known as a halocarbon. While all halogens have the ability to catalyze ozone breakdown, they have an unequal impact on the ozone layer. A catalyst is a compound which can alter the rate of a reaction without permently being altered by that reaction, and so can react over and over again. In this fashion it is estimated that one molecule of chlorine can degrade over 100,000 molecules of ozone before it is removed from the stratosphere or becomes part of an inactive compound. These inactive compounds, for example ClONO2, are collectively called 'resevoirs'. They hold chlorine in an inactive form but can release an active chlorine when struck by sunlight. The relative potency of the different halogens depends a great deal on the stability of the resevior compounds. Hydrogen fluoride, HF, is so very stable that fluorocarbons have relatively no known impact on ozone. Bromine resevoirs, such as HBr and BrONO2, are much more easily broken up by sunlight; causing bromine to be from 10 to 100 times more effective than chlorine at destroying ozone. From 30-60% of bromocarbons released to the atmosphere are man-made (methyl bromide fumigants and halon fire extinguishers) and both compounds will soon be restricted by international agreement.