光是认识世界认识色彩的前提,光是一种客观存在的物质,只有波长在380-780纳米之间的电磁波叫______。
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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