单项选择题

What do we learn from the passage about solar wind activity（）

A．It affects the growth of trees.
B．It has been increasing since the Ice Age.
C．It is determined by the chemicals in the air.
D．It follows a certain cycle.

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单项选择题
What affects the amount of carbon-14 on earth（）

A．The lifecycle of trees.
B．The number of trees.
C．The intensity of solar burning.
D．The quality of air.
单项选择题
Why is there a hole in the weaverbird’s nest（）

A．To let in the sunshine.
B．To serve as its door.
C．To keep the nest cool.
D．For the bird to lay eggs.
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