单项选择题

______ as "the census taker of the sky", Annie Jump Cannon contributed considerably to the field of astronomy.

A．Known
B．Knowing
C．To know
D．Knowledge
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单项选择题
Had the United States built more homes for the poor people in 1955, the housing problem now in some parts of the country ______ so serious.
A．wouldn't be
B．will not have been
C．wouldn't have been
D．would have not been
单项选择题
Wind motion can be observed in the atmosphere by ______ the trails of meteors passing through it.
A．to watch
B．watching
C．watched
D．watch
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