阅读理解。
All of us know trees are useful to man in three very important ways: they give us wood and other
products, they give us shade, and the most important of all, they help to prevent droughts and floods.
In many parts of the world, however, people have not realized the importance of trees to us.
Two thousand years ago, a rich and powerful country cut down its trees to build its warships, with
which it gained itself an empire (帝国). It gained the empire, but without its trees, its soil became hard
and poor. When the empire fell into pieces, the home country found itself faced by floods and hunger.
Even when a government realizes the importance of forests, it is difficult for it to make villagers see
this. The villagers want wood to cook their food, and they can earn money by making things or selling
wood to the town people. They are usually too lazy or too careless to plant and look after new trees. So
unless the government does something to control, and also to educate the people, the forests will slowly
disappear.
1. The most important service of trees is that _______.
A. they help people to get rich
B. they provide people with shade
C. they help to prevent droughts and floods
D. they can be used to build warships
2. The empire fell into pieces because _______.
A. it had lost all of its warships
B. it couldn't provide its people with enough money
C. it didn't ask people to plant many trees
D. its people couldn't make a living
3. Which of the following is NOT true according to the passage?
A. Many people haven't realized the importance of forests.
B. The government should do something to protect the forests.
C. The empire became rich and strong with its powerful warships.
D. People often don't take good care of the new trees they have planted.
4. The forests will disappear if _______.
A. the government realizes the importance of the forests
B. the villagers realize the importance of the forests
C. the government doesn't do something to control or to educate the people
D. the villagers don't cut trees down to make money
5. The writer's purpose in writing the text is _______.
A. to give us advice on how to protect forests
B. to tell the story of the empire two thousand years ago
C. to call on people to do something to protect forests
D. to tell the people to plant as many trees as possible
，已知断裂1 mol B—B吸收167 kJ的热量，生成1 mol B≡B放出942 kJ热量。试写出由C4气态分子变成C2气态分子的热化学方程式：____________________________。
_________________________。