20.设所有被4除余数为k(k=0k(k=0k(k=0,1,2,3)3)3)的整数组成的集合为AkA_{k}Ak,即Ak={x∣x=4n+kA_{k}=\{x\vert x=4n+kAk={x∣x=4n+k,n∈Z}n\in Z\}n∈Z},则下列结论中正确的是((( )))