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【单选题】
对于具有理想约束的质点系,其平衡的充分必要条件是( )。A.
作用于质点系的所有约束力在任何虚位移中所作的虚功的和等于零
B.
作用于质点系的所有约束力在任意一个虚位移中所作的虚功等于零
C.
作用于质点系的所有主动力在任意一个虚位移中所作的虚功等于零
D.
作用于质点系的所有主动力在任何虚位移中所作的虚功的和等于零
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举一反三
【简答题】A．Pretend to be what the company wants. B．Tell the truth, exclude for what you don't want them to know. C．Tell the whole truth. D．Answer it in a roundabout way.
【单选题】The only trouble with young drivers is that
A.
they tend to own sports ear.
B.
they are too proud of their ears.
C.
they soon become like the older ones.
D.
they think they are on the battlefield.
【单选题】Don’t be _____________ if I pretend not to recognize you.
A.
surprise
B.
surprised
C.
surprising
D.
surprises
【单选题】Don’t pretend _____.Look!You are holding your English book upside down.
A.
to read
B.
to have read
C.
to be reading
D.
reading
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