If water is not pumped to the top of a tall tree, and if it is not pushed to the top of a tall tree, then we may ask.. How does it get there? According to the currently accepted cohesion-tension theory, water is pulled there. The pull on a rising column of water in a plant results form. the evaporation of water at the top of the plant. As water is lost form. the surface of the leaves, a negative pressure, or tension, is created. The evaporated water is replaced by water moving from inside the plant in unbroken columns that extend from the top of a plant to its roots. The same forces that create surface tension in any sample of water are responsible for the maintenance of these unbroken columns of water. When water is confined in tubes of very small bore, the forces of cohesion(the attraction between water molecules)are so great that the strength of a column of water compares with the strength of a steel wire of the same diameter. This cohesive strength permits columns of water to be pulled to great heights without being broken.
How many theories does the author mention?
A.One
B.Two
C.Three
D.Four
第1题
患者,男,50岁,临床诊断锁骨下动脉盗血综合征,其最重要的诊断依据是
A、患侧椎动脉彩色及频谱多普勒呈现反向血流
B、患侧上肢脉搏减弱或消失
C、患侧上肢动脉反向血流减弱或消失
D、患侧上肢动脉二维图像无异常
E、患侧椎动脉仍存在舒张期正向血流
第2题
锁骨下动脉窃血综合征最重要的超声诊断依据是:
A、患侧上肢动脉二维图像无异常改变
B、患侧上肢动脉脉冲多普勒频谱反向血流减少或消失,血流速度减慢
C、患侧椎动脉脉冲及彩色多普勒呈现反向血流
D、患侧椎动脉仍存在舒张期正向血流
第3题
锁骨下动脉盗血综合征最重要的诊断依据是
A.患侧上肢脉搏减弱或消失
B.患侧上肢动脉二维图像无异常
C.患侧上肢动脉反向血流减弱或消失
D.患侧椎动脉彩色及频谱多普勒呈现反向血流
E.患侧椎动脉仍存在舒张期正向血流
第4题
锁骨下动脉窃血综合征最重要的超声诊断依据是
A、患侧上肢脉搏减弱或消失
B、患侧上肢动脉二维图像无异常改变
C、患侧上肢动脉脉冲多普勒频谱反向血流减少或消失,血流速度减慢
D、患侧椎动脉脉冲及彩色多普勒血流反向
E、患侧椎动脉仍存在舒张期正向血流
第5题
A.患侧上肢脉搏减弱或消失
B.患侧上肢动脉二维图像无异常改变
C.患侧上肢动脉脉冲多普勒频谱返乡血流减少或消失,血流速度减慢
D.患侧椎动脉脉冲及彩色多普勒呈现反向血流
E.患侧椎动脉仍存在舒张期正向血流
第6题
A.患侧上肢脉搏减弱或消失
B.患侧上肢动脉二维图像无异常改变
C.患侧上肢动脉脉冲多普勒频谱反向血流减少或消失,血流速度减慢
D.患侧椎动脉仍存在舒张期正向血流
E.患侧椎动脉脉冲及彩色多普勒呈现反向血流
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