单项选择题

Nowhere in nature is aluminum found free, owing to its always ______ with other elements, most commonly with oxygen.

A．being combined
B．having combined
C．to combine
D．combined
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单项选择题
This book has been in the works so long that I have lost __ of most of the sources found for me by the staff of the library.
A．trace
B．trail
C．track
D．touch
单项选择题
People under stress have performed ______ feats of strength, like lifting an automobile off an accident victim.
A．specific
B．extraordinary
C．abrupt
D．abnormal
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