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

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热门试题

单项选择题

Two recent papers in Science magazine claim to have found evidence which contradicts the traditional notion that _________.

A．relatively harsh environments are the nurseries of evolution
B．evolution occurred in regions with biological diversity
C．new forms of life come into being in near-shore areas
D．species of birch and redwood originated in the south

单项选择题

Which of the following may be an appropriate title of the passage

A．Darwin's Theory Modified.
B．How Animals Evolve.
C．Evolution in Hard Places.
D．Where Did Large Sea Animals Originate.