The volcanic-eruption theory, like the impact theory, accounts for the presence of iridium in sedimentary deposits; it also (i)_____matters that the meteorite-impact theory does not. Although iridium is extremely rare on the Earth’s surface, the lower regions of the Earth’s mantle have roughly the same composition as meteorites and (ii)_____large amounts of iridium, which in the case of a diapir (iii)_____would probably be emitted as iridium hexafluoride, a gas that would disperse more uniformly in the atmosphere than the iridium-containing matter thrown out from a meteorite impact.