单项选择题

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

A．combined
B．having combined
C．combine
D．being combined
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单项选择题
Every chemical change either results from energy being used to produce the change, or causes energy to be ____________ in some form.
A．given off
B．put out
C．set off
D．used up
单项选择题
In this factory the machines are not regulated ____________ but are jointly controlled by a central computer system.
A．independently
B．individually
C．irrespectively
D．irregularly
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