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北极地区的平均气温比南极地区高，你认为根本的原因是（    ）

下列事例的物态变化过程中，属于放热的是（　　）

阅读理解

A report by UNEP联合国环境规划署（United Nations Environment Programme） says that by 2050, the world's population is expected to reach 9.6 billion and feeding us all will require at least 60% more food than we have ever produced before. What's more, if we can't find out a way to do with it, food shortages (短缺) could be a big problem.

A main purpose of the report is to make people be aware of the importance of this challenge. In the past, food production increase relied on the practices like cutting down trees, using chemicals and wasting water. Today we understand the environmental price of those techniques.

“The question now is how to produce much more food and at the same time avoid all the environmental problems that we had in the past,” Searchinger, a famous scientist said.

We need to develop new ways to produce food. We can no longer cut down trees. Instead, we'll have to protect water and land, and reduce air pollution.

The report also mentions the importance of reducing food loss and wasteAccording to the United Nations, the waste adds up to one-third of all the food we produce —an amount that could feed an additional 2 billion people.

The report also suggests that we make changes in our diet. “You have to eat more efficiently (有 效地),” said Searchinger, “and above all that means eating less meat, and especially reducing the growth of beef.”That's because raising cows is an inefficient way of producing food. Of all the energy that goes onto feeding cows, only about 1% is converted (转换) into food for people.