设R1是A1={1,2}到A2=(a,b,c)的二元关系,R2是A2到A3={}的二元关系,R1= {<1,a>,<1,b>,<2,c>}, R2={<a,>,<b,>}求R1R2的集合表达式.
A campus emergency ______ occur at any time of the day or night, weekend, or holiday, with little or no warning.
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