单选题 .  ①Our study revealed that nest-guarding long-tailed skinks (a species of lizard) homed (returned to their nests) more successfully when displaced shorter distances. ②There are two reasons why homing success rates decreased with increasing displacement distance. ③One possibility is that females were simply displaced too far to find their way home. ④However, this is unlikely given that some individuals managed to find their way home from each distance we used. ⑤The second possibility deals with trade-offs between the risks associated with making a long return trip and the benefits of returning. ⑥Animals should expend energy only when the associated costs are low. ⑦As reptiles increase the time spent moving, their daily energy expenditure increases dramatically. ⑧The energetic costs of returning home and the chances that the eggs will have been preyed upon during the return trip both increase substantially as displacement distance increases. ⑨For example, the 130 hours (5.5 days) that female skinks spent returning from a distance of 300 meters is sufficient for an egg-eating snake to locate and prey upon the entire clutch. ⑩However, females with larger clutches were more likely to home at distances over 50 meters. 25.  The primary purpose of the passage is to ______