单项选择题

Which one of the following was used to determine the structure of the DNA molecule ( )

A．Transmission(透射) electron microscope
B．Scanning electron microscope
C．Differential centrifugatioin(差速离心)
D．X-ray crystallography
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单项选择题
Plant guard cells(保卫细胞) have a higher [C1-] than the outside environment. The most likely explanation for this is( ).
A． Water leaves the cell by osmosis.
B． C1- is diffusing through transmembrane channels.
C． C1- diffusion is aided by a mobile carrier.
D．An active transport symport transports C1-/H+.
E．None of the above
单项选择题
在下列组织中, 紧密连接特别重要的有( )
A．Stomach(胃) and kidneys
B．Smooth muscle
C．Heart muscle
D．Intestinal epithelial tissue(肠表皮组织)
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