A group of unusual meteorites was found in Shergotty, India. Their structure indicates that they originated on one of the geologically active planets, Mercury, Venus, or Mars. Because of Mercury' s proximity to the Sun, any material dislodged from that planet' s surface would have been captured by the Sun, rather than falling to Earth as meteorites. Nor could Venus be the source of the meteorites, because its gravity would have prevented dislodged material from escaping into space. The meteorites, ttherefore, probably fell to Earth after being dislodged from Mars, perhaps as the result of a collision with a large object. The argument derives its conclusion by ['eliminating competing alternative explanations', 'contrasting present circumstances with past circumstances', 'abstracting a general principle from specific data', 'offering a counterexample to a theory']