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【单选题】

How did scientists discover the source of lead pollution in Greenland( )?

A.
By yzing the data published in journals like Nature and Ambio.
B.
By observing the lead accumulations in different parts of the arctic area.
C.
By studying the chemical elements of soil and snow in Northeastern America.
D.
By comparing the chemical compositions of leaded gasoline used in various countries.
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【单选题】How did scientists discover the source of lead pollution in Greenland?

A.
By analyzing the data published in journals like Nature and Ambio.
B.
By studying the chemical elements of soil and snow in North eastern America.
C.
By comparing the chemical compositions of leaded gasoline used in various countries.
D.
By observing the lead accumulations in different parts of the arctic area.

【单选题】Read the introduction [paragraphs 1-6]. How is bouldering different from lead climbing?

A.
In bouldering, people race up tall walls, and in lead climbing people have to be careful not to fall because they do not have ropes.
B.
In bouldering, people follow paths on low walls, and in lead climbing people see how high they can climb in six minutes.
C.
In bouldering, people use ropes to stay safe, and in lead climbing people climb sideways across a low wall.
D.
In bouldering, people climb indoors, and in lead climbing people climb outdoors as long as the weather is good.

【单选题】下列哪种继电器用于给出指示信号。()

A.
时间继电器
B.
信号继电器
C.
中间继电器
D.
电流继电器

【单选题】下列哪种继电器用于给出指示信号。( )

A.
过电流继电器
B.
时间继电器
C.
功率方向继电器
D.
信号继电器