单项选择题

Listen to the statement and circle the appropriate letter. When the number of the sea urchins decreases, the kelp will _________ .

A．be extinct.
B．flourish again.
C．keep unchanged.
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单项选择题
Which of the following presents the Domino effect in ecological community
A．The symbiotic relationship between different species.
B．The extinction of the hippopotamus affects the existence of other species dependent on them.
C． The descent of species.
单项选择题
This ecological balance among sea urchins, sea otters, and kelps became __________ during the nineteenth century.
A．upset
B．satisfactory
C．neutral
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