Random ra=new Random ();
            int[,] a = new int[3, 4];
            int[,] b = new int[3, 4];
            int[,] c = new int[3, 4];
            ArrayList ar=new ArrayList() ;
            for(int i=0;i<3;i++)
            {
                for(int j=0;j<4;j++)
                {
                    a[i, j] = ra.Next(10, 100);
                }
            }
            ar.Add(a);
            for (int i = 0; i < 3; i++)
            {
                for (int j = 0; j < 4; j++)
                {
                    b[i, j] = ra.Next(10, 100);
                }
            }
            ar.Add(b);
            for (int i = 0; i < 3; i++)
            {
                for (int j = 0; j < 4; j++)
                {
                    c[i, j] = ra.Next(10, 100);
                }
            }
            ar.Add(c);

//ar 里加入了三个二维数组，怎样把它们一一取出。