题目内容：

    Rivers are the veins of the Earth, transporting the water and nutrients (营养物) needed to support the planet’s ecosystems, including human life. While many nutrients are essential to the survival of life, there is one element transported by water in rivers that holds the key to life and to the future of our planet — carbon.

Carbon is everywhere and understanding the way it moves and is either released or stored by the Earth system is a complex science in itself. Carbon starts its journey downstream when natural acid rain, which contains carbon dioxide from the atmosphere,  melts minerals in rocks. This helps transform carbon dioxide to bicarbonate (碳酸氢盐) in the water that then flows in our rivers. This is a very long process, which is one of the main ways carbon dioxide is removed from the atmosphere. Carbon is transported by rivers to oceans and once that carbon reaches the ocean, it is stored naturally in deep sea sediments (沉淀物) for millions of years.

As carbon travels down a river, different processes may impact whether it continues to flow downstream or whether it is released into the atmosphere. For example, human engineering, like extensive dam construction, will result in dramatic changes to how water and sediments travel down the river. Some carbon that fails to reach the sea may return to the atmosphere in some way, which causes more warming.

Earth’s climate is closely related to the carbon cycle. We all know about the essential role of plants in consuming carbon dioxide, but do we know enough about rivers? Changing the chemistry and the course of rivers may have significant impacts on how they transport carbon. Remember: wherever we live, we all live downstream.

1.Where is the carbon in rivers originally from?

A.The atmosphere. B.The rocks.

C.The acid rain. D.The upstream areas.

2.Why is human engineering mentioned in Paragraph 3?

A.To show how important to life carbon is.

B.To explain how necessary it is to build dams.

C.To show how a natural process is interrupted.

D.To explain how humans fight global warming.

3.What does the author want to convey in the last paragraph?

A.We’d better move upstream to live.

B.We should protect plants along rivers.

C.We’d better seek more help from plants.

D.We should be cautious about river management.

4.What is the best title for the text?

A.What Humans Do with Rivers

B.How Rivers’ Transporting Carbon Counts

C.What the Carbon Cycle Means to Us

D.How Living Downstream Affects the Earth