在研究下列问题中，能够把研究对象看作质点的是（   ）

若圆锥的侧面展开图为半圆，则该圆锥的母线l与底面半径r的关系是

   There are four seasons in a year. They are spring, summer, autumn and winter. For all my friends, every season is good.

   Wang Dong likes flying kites and climbing hills. She says when spring comes, the days are getting warmer and longer. And everything begins to grow. The wind is not very strong. It’s the best time to fly kites. And when you climb the hills, you can enjoy yourself. So she often says spring is her favourite season.

   “Swimming is my favourite sport,”says Anne. “When summer comes, I often go swimming in the rivers or lakes with my friends. ”You can enjoy yourself in the water. And you can swim like a fish there.

   “I often go climbing hills with my father during the long vacation of the National Day, ”says Liu An. “My father often tells me that I can see lots of beautiful things if I stand on the top of a hill. ”So he likes autumn best of all.

   Bob says he likes winter very much. He says, “I’m very happy when I am playing with snow.”He is making a snowman now.

    November 24,2001 Thursday

    Wild animals play an important role in our environment. However, many human activities have put them in serious danger of extinction. By the year 2025, one fifth of the world's animal species may be extinct.

提示要点：1）描述动物的现状；

2）应该采取的措施；

3）谈谈自己的看法。

注意：1）词数100左右；

2）可适当增加细节，以使行文连贯。