在光栅光谱中,假如所有偶数级次的主极大都恰好在每缝衍射的暗纹方向上,因而实际上不出现(缺级),那么此光栅每个透光缝宽度a和相邻两缝间不透光部分宽度b的关系为:()
A campus emergency ______ occur at any time of the day or night, weekend, or holiday, with little or no warning.
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