using System;

using System.Text;

using System.Linq;

​

public class Program {

    public static void Main() {

        string[] names = { "Mark", "Alice", "Bob" };

        Console.WriteLine("A) "

            + names.Aggregate("", (sb, x) => sb + ',' + x).ToString().TrimStart(','));

        Console.WriteLine("B) "

            + names.Aggregate("", (sb, x) => $"{sb},{x}")

            .ToString().TrimStart(','));

​

        Console.WriteLine("C) "

            + names.Aggregate(new StringBuilder(),

            (sb, x) => sb.Append(x).Append(','))

            .ToString().TrimEnd(','));

​

        Console.WriteLine("D) "

            + names.Skip(1).Aggregate(

            new StringBuilder().Append(names.FirstOrDefault()),

            (sb, x) => sb.Append(',').Append(x)).ToString());

​

        Console.WriteLine("E) "

            + string.Join(',', names));

    }

}