Even if trees cannot walk, they are still on the move. In parts of the Arctic, entire forests are moving northward. Across the Arctic, temperatures are rising faster than anywhere else in the world. As that happens, the tree line that marks where forests stop and the treeless tundra  starts has been shifting northward. Trees growing along the tree line must protect themselves from the cold wind. To do _ , plants tend to grow horizontal  branches low to the ground. The energy it takes for trees to grow this way means they don't have enough energy to make seeds. But as Earth's climate has been warming, trees no longer have to just grow horizontally. Many can instead grow up toward the sky. This takes less energy. And with all the leftover energy. these trees have started producing more seeds. This happens especially in places where the white spruce  grows. White spruce, which is a North American tree, is quite able to produce a lot of seeds, which can move long distances in the wind. When wind-blown seeds end up on the tundra beyond the tree line, they eventually can sprout   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 temperature has helped more trees 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. What is the main cause of the trees in the Arctic moving northward?
The rising temperature. The shrinking of northern tundra. The fierce and cold wind. The abundant resources in the north.
Question: Even if trees cannot walk, they are still on the move. In parts of the Arctic, entire forests are moving northward. Across the Arctic, temperatures are rising faster than anywhere else in the world. As that happens, the tree line that marks where forests stop and the treeless tundra  starts has been shifting northward. Trees growing along the tree line must protect themselves from the cold wind. To do _ , plants tend to grow horizontal  branches low to the ground. The energy it takes for trees to grow this way means they don't have enough energy to make seeds. But as Earth's climate has been warming, trees no longer have to just grow horizontally. Many can instead grow up toward the sky. This takes less energy. And with all the leftover energy. these trees have started producing more seeds. This happens especially in places where the white spruce  grows. White spruce, which is a North American tree, is quite able to produce a lot of seeds, which can move long distances in the wind. When wind-blown seeds end up on the tundra beyond the tree line, they eventually can sprout   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 temperature has helped more trees 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. What is the main cause of the trees in the Arctic moving northward?
A. The rising temperature.
B. The shrinking of northern tundra.
C. The fierce and cold wind.
D. The abundant resources in the north.
Answer: