Enteroliths are calcifications that form in the gut of a horse. The stones can cause considerable morbidity and mortality. A study was conducted to investigate the factors (such as diet and environment) that may be related to the formation of enteroliths. The study contained seven stallions; their ages (in years) are as follows: 10 20 4 13 21 16 16 The youngest stallion, at 4 years old, is: not an outlier, because it is a positive value. O an outlier, because it is the minimum. O an outlier, because it is more than 15 years below the lower quartile. not an outlier, because it is less than 15 years below the lower quartile