Beavers are known for building dams, but a new study shows they might also be unexpected heroes in the fight against climate change. An international research team, led by the University of Birmingham, studied a stream in northern Switzerland where beavers have been active for over ten years. The scientists found that wetlands shaped by beavers store carbon up to ten times faster than similar areas without beaver dams. Over 13 years, the site captured about 1,194 tonnes of carbon. That equals 10.1 tonnes of carbon dioxide per hectare each year.

This process happens mainly because beaver dams slow down moving water. As the water calms, fine materials — known as sediments (沉淀物) — sink to the bottom. Dead plants and pieces of wood also settle. Over time, layers of these materials build up, trapping carbon deep underground. The research showed that such sediments can hold up to 14 times more inorganic (无机的) carbon and eight times more organic carbon than nearby forest soils. Nearly half of the long‑stored carbon came from deadwood in riverside forests.

The team also measured greenhouse gases like methane (CH₄). Methane release was very low, contributing less than 0.1 % of the total carbon budget. This means the wetland remains a net carbon sink. Although dry summers can cause some carbon dioxide to escape, the wetland stores far more carbon than it releases over a full year.

Dr Joshua Larsen, the lead senior author of the study, said that beavers do not simply change the landscape — they transform the way carbon dioxide moves through it. By expanding wetlands, they turn ordinary streams into powerful carbon sinks.

1.1. What advantage do beaver‑shaped wetlands have over areas without beaver dams?

A Larger size.

B Faster carbon storage.

C Less water flow.

D Higher methane release.

解析：选B。B细节理解题。根据第一段中“The scientists found that wetlands shaped by beavers store carbon up to ten times faster than similar areas without beaver dams.”可知，这类湿地储存碳的速度最高可达十倍，对应选项B“更快的碳储存”。A项“面积更大”文中并未提及；C项“水流更少”虽是海狸坝带来的改变，但并非题目所问的相比无坝区域的直接优势，且原文对比的是碳储存速度，而非水流大小；D项“甲烷释放更多”与第三段中甲烷释放极低的描述完全相反。故选B。

2.2. What is the second paragraph mainly about?

A Why beavers build dams.

B What forest soils contain.

C Where deadwood comes from.

D How carbon is stored by dams.

解析：选D。D主旨大意题。第二段首先点明过程发生的原因——海狸坝减缓水流，接着详细描述细颗粒物、枯死植物和木块如何沉降并层层堆积，将碳封存在地下，最后用数据说明这类沉积物比森林土壤含有更多的无机碳和有机碳，并指出近一半的长期储存碳来自河岸森林的死木。整段都在解释海狸坝如何一步一步实现碳储存，因此D项“碳是如何被坝储存的”准确覆盖了段落主旨。A项“海狸为什么建坝”在该段并未分析；B项“森林土壤含有什么”只是作为对比出现，并非核心内容；C项“死木来自何处”仅在段末提及，同样无法概括全段。故选D。

3.3. In Paragraph 3, the underlined phrase “a net carbon sink” most probably means a place that ______.

A gives off more carbon than stored

B stores more carbon than it releases

C releases only methane, nothing else

D dries up completely in summer

解析：选B。B词义猜测题。第三段先指出甲烷释放量极低，不到总碳收支的0.1%，随后用“This means the wetland remains a net carbon sink”引出结论，紧接着用“Although dry summers can cause some CO₂ to escape, the wetland stores far more carbon than it releases over a full year”对这一结论进行解释。可见，“net carbon sink”指的就是储存的碳远远多于释放的碳，即净碳汇，故选B。A项“释放的碳多于储存的碳”属于碳源，与文意相反；C项“只释放甲烷，没有其他”与事实不符，因为该段也提到干旱夏季会逸出部分CO₂；D项“在夏季完全干涸”曲解了原文，原文只是说干旱夏季可能导致一些CO₂释放，并未说湿地完全干涸。故选B。

4.4. What does the figure 1.2–1.8% in the last paragraph indicate?

A Beavers flood the land.

B Beavers need machines.

C Beavers can reduce emissions.

D Beavers work best in dry seasons.

解析：选C。C推理判断题。最后一段指出，如果海狸回归瑞士所有适宜的洪泛区，它们能够帮助抵消该国年碳排放量的1.2%–1.8%，而且这一过程无需人类直接付出努力。能够“抵消碳排放”即意味着海狸的活动可以带来碳减排的效果，因此C项“海狸能够减少排放”是对该数据的合理推断。A项“海狸淹没土地”在该段并未涉及；B项“海狸需要机器”与原文“without direct human effort”完全矛盾；D项“海狸在旱季工作效率最高”也无从推知，原文虽提到旱季会有一些CO₂逸出，但未对旱季与其他季节的碳汇效率进行对比。故选C。