单项选择题

The students in Louisiana ______.

A．worked together to find solutions to a problem over the Internet
B．cleaned up a polluted stream across their home town
C．received a grant of ＄ 50,000 for their project
D．lobbied the local and state political leaders
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单项选择题
On which of the following assumptions is based the belief that tree-ring thickness shows links between solar periodicity and terrestrial climate
A．Solar-activity cycle existed in its present form during the period in question.
B．Average tree-ring thickness varies from species to species.
C．Tree-ring thickness varies with changes in terrestrial climate.
D．Both terrestrial climate and solar-activity cycle affect treering thickness.
单项选择题
The author focuses primarily on ______.
A．two competing scientific models concerning the sun's magnetic field
B．an overview of some recent scientific developments in solar physics
C．the reasons why a problem in solar physics has not yet been solved
D．the difficulties involved in linking terrestrial climate with solar activity
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