单项选择题

Animal migration is a seasonal or periodic movement of animals in response to changes in climate or food ______, or to ensure reproduction.

A．alternation
B．availability
C．utilization
D．capacity
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单项选择题
Our research has focused on a drug which is so ______ as to be able to change brain chemistry.
A．influential
B．powerful
C．monstrous
D．rigorous
单项选择题
The concept of ______ grouping of people with similar interests and abilities was very popular among educators.
A．segregated
B．integrated
C．homogeneous
D．heterogeneous
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