On the nights immediately following the mysterious Tunguska event, which destroyed a tract of Siberian wilderness in 1908, eyewitnesses reported seeing noctilucent clouds -- brilliant night-visible clouds made up of ice particles that form rarely and only at very high altitudes. Recently, noctilucent clouds have been observed on the nights following launches of rockets that release large amounts of water vapor into the upper atmosphere. This shows that it was a comet impact and not the impact of an asteroid that caused the destruction in Siberia. The argument depends on assuming which one of the following? ['Comets but not asteroids release large amounts of water vapor into the upper atmosphere when they collide with Earth.', 'The fact that noctilucent clouds are made of ice particles in the upper atmosphere was only recently discovered.', 'Comets collide with Earth more frequently than asteroids do.', 'Noctilucent clouds are visible for many consecutive nights following the release of water vapor into the upper atmosphere.']