The lion says that he is________So he asks all the animals to come and listen to his last wishes.

    The cat comes to the lion's cave(洞穴). He stands there and goes in. Then a sheep goes in. Before she comes out, a rabbit goes in to hear the last wishes of the king of animals.

    But soon the lion looks well again, and goes out of his cave. He sees a fox(狐狸) waiting outside. “Why don't you come in?” asks the lion to the fox.

    “I beg your pardon,” says the fox, “I see many animals go into your cave, but nobody comes out. Before they come out again, I will wait outside.”

如图，以△ABC的边AB为直径作⊙O，⊙O与边BC的交点D恰为BC边的中点，过点D作DE⊥AC于点E．

（I）求证：DE是⊙O的切线；

（Ⅱ）若∠B=30°，求$\frac{AE}{DC}$的值．

青藏地区突出的自然环境特征是（    ）