beyond the tree line，they eventually can sprout (发芽) new trees.This explains how a forest can move.Of course，the process would work only if the tundra were warm enough.But

　　in recent years，the whole planet has been warming.

　　New trees will provide shelters for some snow.Keeping

　　the sun’s rays from making the white surface disappear.Instead，the trees absorb the sun’s heat.This warms the surrounding air.The extra warmth encourages even more trees

　　to produce seeds.That further boosts a forest’s ability to expand.In

　　addition，more

　　trees

　　will

　　trap

　　more

　　snow，preventing much of it from being blown away.Snow can trap heat in the soil below，which encourages trees to grow.The recent rise of temperatures has helped more trees grow past the

　　tree line.People worry about impacts on the animals that

　　depend on frozen conditions for food and shelter.

　　1．What is the main cause of the trees in the Arctic moving northward?

　　A．The shrinking of northern tundra.

　　B．The rising temperature.

　　C．The fierce and cold wind.

　　D．The abundant resources in the north.

　　2．What does the underlined word “ this”

　　in Paragraph 2 mean?

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