The size of the spleen is a good indicator of how healthy a bird is: sickly birds generally have significantly smaller spleens than healthy birds. Researchers found that, in general, birds that had been killed by predators had substantially smaller spleens than birds killed accidentally. Which one of the following is most strongly supported by the information above? ['Sickly birds are more likely than healthy birds to be killed by predators.', 'Small spleen size is one of the main causes of sickness in birds.', 'Most birds with smaller than average spleens are killed by predators.', 'Predators can sense whether a bird is sick.']