When a planetary system forms, the chances that a planet capable of supporting life will be formed are high. The chances that a large planet the size of Jupiter or Saturn will be formed, however, are low. Without Jupiter and Saturn, whose gravitational forces have prevented Earth from being frequently struck by large comets, intelligent life would never have arisen on Earth. Since planetary systems are unlikely to contain any large planets, the chances that intelligent life will emerge on a planet are, ttherefore, low. Knowing which one of the following would be most useful in evaluating the argument? ['how likely it is that planetary systems containing large planets will also contain planets the size of Earth', 'whether large comets could be deflected by only one large planet rather than by two', 'how high the chances are that planetary systems will contain many large comets', 'whether all planetary systems are formed from similar amounts of matter']