Which of the following are difficult to explain in terms of natural selection?

• A. Male peacocks evolve tail feathers that would appear to make them more rather than less vulnerable to predators.
• B. Male deer evolve antlers that are not used to defend themselves against predators.
• C. A bird issues a warning cry that puts it at greater risk of being noticed by a predator.
• D. Some traits appear to have no adaptive value.