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【单选题】
Which of the following statements is correct? (1) On a section of the bar, if the axial force N is positive(tension), the normal stress σ at each point is also positive(all are tensile stress). (2) On a section of the bar, if the normal stress( σ ) at each point is positive, the axial force N must also be positive. (3) On a section of the bar, if the axial force N is not zero, the normal stress( σ ) at each point is not zero. (4) On a section of the bar, if the normal stress( σ ) at each point is not zero. then the axial force N must not be zero.A.
(1)
B.
(2)
C.
(3), (4)
D.
all are correct
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