A、Keeping its feathers waterproof. B、Keeping its feathers cool in summer. C、Helping communicate with the same species. D、Keeping

Birds spend so much time smoothing, cleaning and arranging their feathers that it’s not surprising this behaviour, called preening, is associated with pride and vanity. But stop a minute and think about what feathers do for a bird. Besides allowing it to fly, feathers keep the bird warm in the winter and cool in the summer. They shed water in the rain, and they are used to communicate with other birds of the same species. A bird with dirty feathers will not live long.
    Birds use their beaks to clean and straighten their feathers and to spread oil over them. They get the oil from their skin.[17]The oil helps keep the feathers waterproof and flexible.
    Birds also like to take baths to keep clean. They don’t always take them in water.[18]Chickens, for instance, like nothing better than to scratch up some dust with their feet and then lie down and flap the dust all over their bodies. This probably helps get rid of parasites on their skin. A quick shake then sends the dust flying from their feathers, leaving them clean and comfortable.
16. What is the passage mainly talking about?
17. What is the function of the oil on bird’s skin?
18. What do chickens use to take baths?