My name is Zhang Peng. I have a friend. His name is John. He has a grandpa and a grandma. They are seventy years old. They are at home. John's father works in a school. His mother is a doctor. John is a good student. They are very busy. They all get up early in the morning. John often helps his grandparents cook breakfast. There is a big garden and many young trees in front of his house. We can play games there. It's great.

2010年4月，墨西哥湾发生了严重的石油泄漏事件，若油区内的鱼类死亡，原因可能是（  ）
①油膜覆盖海面，海水含氧量减少
②缺少阳光，水生植物无法进行光合作用，鱼类缺乏饵料 
③海水污染 
④油膜太厚，鱼类难以游动

将一个底面直径是10厘米，高为36厘米的圆柱体锻压成底面直径为20厘米的圆柱体，在这个过程中不改变的是（　　）