关于四则运算的程序代码

Random random = new Random();
            for (int i = 0; i < 30; i++)
            {
                int[] a = new int[30];
                int[] c = new int[30];
                a[i] = random.Next(0, 100);
                c[i] = random.Next(0, 100);
                String b;
                if (a[i] % 4 == 0)
                    b = "+";
                else if (a[i] % 4 == 1)
                    b = "-";
                else if (a[i] % 4 == 2)
                    b = "*";
                else
                    b = "/";
                richTextBox1.AppendText(a[i].ToString() + (b) + c[i].ToString()+("=    "));
            }