单项选择题

A．They have difficulty identifying these cells.
B．They aren’’t sure how these cells work.
C．They’’ve learned how to reproduce these cells.
D．They’’ve found similar cells in other species.
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单项选择题
The power of the female heart remains undiminished between the ages of 18 and 70 probably because _________.
A．women almost lose no contractile muscle cells in the walls of their hearts
B．women’’s oestrogen has a protective effect on their heart
C．the size of their heart chambers is different from men’’s
D．the thickness of the female heart’’s muscular wall is different from men’’s
单项选择题
The words work and play are generally used to refer to day and night.
A．Y
B．N
C．NG
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