假设有两个天体可分别看成是半径为R₁和R₂的质量分布均匀的球体，若两个天体的密度相等，R₁=3R₂，则两个天体的第一宇宙速度之比v₁：v₂为（　　）

下列关于酶的叙述，正确的是（    ）

    In the middle of a dark forest, there lives an owl(猫头鹰). All the animals know that he is the wisest(聪明的) among all of them. So they often go to him and tell him their troubles. Then the owl always teaches them what they should do.
     One day, a little bird goes up to the owl. She is crying. “what's wrong with you?” asks the owl.
    “I'm not happy at all. I don't want to be a bird,” the bird says.
   “Why don't you want to be a bird?” asks the owl.
   “I am so small, weak and useless,” she cries. “I want to be big and strong like the lion. He is very important, but I am not”
     Then the owl whispers(耳语) something in her ear. At once the bird dries her tears and goes away. One week later, the bird came back. “Oh, thank you very much. You are a very wise owl,” she says to the owl. “I am very happy now. Every day, I sit on top of the tallest tree and watch for(监视) lions and tigers. When they come near, I shout at the top of my voice. Then my friend all run and hide. They are very thankful to me. I am useful and important now”
    The wise owl smiles and says, “No one in this world is useless.”

新中国成立初期，蝗虫波及7省2市，灾情十分严重。1952年，国家组建了中国科学院昆虫研宄所和昆虫生态学研究室，委派中国生态学奠基人马世骏先生带领一批科学家和技术人员技出根治蝗灾的办法。延续了两千多年的蝗灾，在新中国成立后的短短几年内便得到有效防，不能不说是一个奇迹。科学家和技术人员的努力，国家的高度关注和支持，社会各方面的配台，是治蝗成功的关键因素。自此我们可以认识到（   ）

They were trying to remove harmful ___________ from cigarettes.