Rainforests like the Amazon are important for mopping up CO2 from the atmosphere and helping to solve global warming. Currently the trees in the Amazon take in around 500 million tons of CO2 each year: equal to the total amount of CO2 given off in the UK each year. But how will the Amazon react to the future climate change? If it gets drier, will it survive and continue to draw down CO2? Scientists hope that they will be able to learn in advance how the rainforest will manage in the future by understanding how rainforests reacted to climate change in the past.

　　Unfortunately, collecting information is incredibly difficult. To study the past climate, scientists need to look at fossilized pollen（花粉）kept in lake mud, Going back to the last ice age means drilling down into lake sediments（沉淀物）, which requires specialized equipment and heavy machinery. There are very few roads and paths, or places to land helicopters and aeroplanes.

　　Rivers tend to be the easiest way to enter the forest, but this still leaves vast areas between the rivers completely unsampled（未取样）. So far, only a handful of cores have been drilled that go back to the last ice age and none of them provide enough information to prove how the Amazon forest reacts to climate change.

　　25. How do scientists study the past climate change?

　　A. By predicting the climate change in the future.

　　B. By drilling down deep into land sediments.

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