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【单选题】
质点系质心在某轴上的坐标不变，则（）A.
质点系质心的初速度必为零
B.
质点系各质点的初速度在此轴的分速度必为零
C.
作用于质点系上所有外力的矢量和必恒等于零
D.
开始的初速度并不一定等于零，但质点系上所有外力在此轴上投影的代数和必恒等于零
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A.
offered
B.
offers
C.
is offered
D.
was offered
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A.
exhausting
B.
demonstrating
C.
using
D.
exhibiting
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A.
不管产权是否明确，只要交易成本为零
B.
不论产权是否明确，交易成本是否为零
C.
只要产权明确，且交易成本为零
D.
只要产权明确，不管交易成本有多大
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A.
than
B.
for
C.
with
D.
to
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A.
on
B.
of
C.
about
D.
in
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