单项选择题

The author provides an example of smallpox inoculation in the last paragraph in order to show that _________.

A．inoculation reflects the medical interaction between west and east
B．Asian people learned immunization earlier than the Europeans
C．the Europeans learned immunization earlier than Asian people
D．immunization was practiced on people of upper class for some time in the eighteenth Britain

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单项选择题

What conditions are needed for shrubs to become established

A．Ferns must take root.
B．The ground must be covered with grass.
C．More soil must accumulate.
D．Smaller plants must die out.

单项选择题

In a primary succession , what makes it possible for mosses to take root

A．The type of rock.
B．The amount of sunlight.
C．The amount of moisture.
D．The effect of lichens.