using System;

using System.Text;

​

public class Program

{

    public static void Main()

    {

        var prefix = "C";

        var padLeftSize = 5;

        var sb = new StringBuilder();

        for (var i = 0; i < 15; i++) {

            sb.Clear();

            sb.Append(prefix);

            sb.Append(i.ToString().PadLeft(padLeftSize, '0'));

            Console.WriteLine(sb.ToString());

        }

    }

}

​