单项选择题

_______ in the color of sea water from blue to green seem to be caused by high or low concentration of salt.

A．Variations
B．Evaporations
C．Explorations
D．Explosions
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It is desirable that students should acquire some knowledge of the writers whose _______ are placed before them for study.
A．materials
B．manufactures
C．products
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We have a high regard for Mr. Smith because he always _______ his principles.
A．stands up to
B．lives up to
C．puts up for
D．keeps up with
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