执行如图所示的程序框图，输出结果是4．若$a_0\in \left\{1,2,3\right\}$，则a₀所有可能的取值为（  ）

对比实验是运用比较的方法来揭示事物的性质或变化规律的一种实验方法。课本中设计的下列实验属于对比实验的是（  ）

Reading is an activity people enjoy a lot in their free time. Some like reading newspapers, and others enjoy novels or comic books. I like reading about the lives of great poeple. This always gives me a lot of ideas on how to make my own life better.
Great people are remembered not because they were handsome or beautiful, but because they did not give up when their lives were difficult. They used every opportunity to change their lives and make the world better.
One good example is Orville and Wilbur Wright, the two brothers who invented the airplane. The plane has made the world into a small village. Hard work, not good luck, is the reason why the Wright Brothers could invent this convenient machine and become remarkable people. Today we wtil remember them when we see planes in the sky.
When I feel sad, stories of great people always help me feel better. This is why I enjoy reading about great people’s lives.

日本派专家帮助中国防治酸雨，韩国公民自发组织到北京西郊植树，这说明了（    ）

①环境问题可能会影响到周边国家和地区

②污染物质具有残留性

③发达国家将污染工业转移到发展中国家

④解决环境问题需要国际协作