The Raven and the Swan（在线收听）

    A raven envied a swan the whiteness of her plumage；and，thinking that her beauty was owing to the water in which she lived，deserted the altars where he used to find his livelihood，and betook him-self to the pools and streams．There he plumed and dressed himself and washed his coat，but all to no purpose，for his plumage remained as black as ever，and he himself soon perished for want of his usual food．Change of scene is not change of nature．Exercises：
    回答下列问题：
    ① According to the raven，why did the swan look beautiful？
    ② Where did the raven want to betake himself？
    ③ Did the raven's plumage change？
    ④ What can we learn from the fable？