= Solution
For <fluorine>, the key is \b[chemical sequestration], not an inability of free F to react. <Fluorine> atoms readily form very stable <hydrogen fluoride> through reaction with hydrogen-containing gases. Breaking <hydrogen fluoride> or regenerating a sustained free-fluorine <radical> population under ordinary stratospheric conditions is difficult, so an efficient analogous catalytic family is not maintained.
For <iodine>, the traditional approximation is \b[limited delivery to the <stratosphere>]. Many <iodine> source gases are short lived in the <troposphere>, and their reactive products are efficiently taken up or removed before slow large-scale ascent. This historically motivated omitting <iodine> from the major stratospheric <chlorine>/<bromine> budget. It does not establish that <iodine> atoms are intrinsically ineffective catalysts, or that stratospheric <iodine> is exactly zero.
Indeed, the blanket omission needs qualification even for this exam's date. Aircraft observations published in 2020 detected both <IO> and particulate <iodine> in stratospheric air and inferred continuing multiphase recycling and <ozone> loss. Gas/particle partitioning and measurement sensitivity must be considered together; a small gas-phase <concentration> alone can miss an active reservoir. Thus the traditional supply argument is the intended contrast with <fluorine>'s stable-HF sink, but \b[zero stratospheric <iodine> chemistry is not supported by those observations].<Quantitative detection of iodine in the stratosphere by Theodore K. Koenig and others>[PNAS 117 (2020), pp. 1860–1866, Abstract and Significance: aircraft IO/particulate-iodine measurements and nonzero stratospheric injection]{ref}
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