对某一调查对象在一个较长的时间内的特征变化进行调查,目的是了解研究对象前后的变化和差异情况,这是教育调查研究中的 ( )
Goods are carried by several of transport--on road or rail, by sea or air.
A campus emergency ______ occur at any time of the day or night, weekend, or holiday, with little or no warning.
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