From the first use of the rocket to carry satellites into space to the setting up of space stations,human beings have been putting great effort into space research. And so far,we have achieved many successes. But there are still numerous tasks in front of us and we should not cease trying to progress. The international space station is an important step we should take in space exploration. It is not only helpful but also essential. It provides a proper space environment for many experiments that we have wanted to do for a long time. It is also a base for the observations of the earth and the universe. It could also be an important base for later travel to the moon and Mars. In a word,if we want to explore space more,the first thing we should do is to set up a space station. As the space station costs a lot of money,it is hard for one country alone to establish one. The USA seems to be the only country that has the ability to build a space station alone,and it has tried to do so,but not very successful. So it  _ many other countries to work on it together. Though it is still extremely expensive,it is much cheaper than doing it alone. It is really a job that needs everyone's effort and will benefit everyone. But even so, the funds needed are still a big problem. As for the USA,it seems that it has too many things to spend its money on. Although it is the richest country in the world,it has much debt every year. It has to make arrangements about its finance,and try to find a balance in all these issues. Which of the following aspects about building the international space station is NOT mentioned in the passage?
Difficulties. Cooperation. Significance. Specific arrangements.