using System;

using System.Linq;

public class Program

{

    public static void Main()

    {

        string[] ab = { "aa", "ab", "bb", "bc", "cd", "ce", "" };

        var resultOne = ab

            .Where(x => x.StartsWith("a", true, System.Globalization.CultureInfo.CurrentCulture) 

                || x.StartsWith("b", true, System.Globalization.CultureInfo.CurrentCulture))

            .SelectMany(x => x)

            .ToArray();

        Console.WriteLine(resultOne);   

        var resultTwo = ab

            .Where(x => x.StartsWith("a", true, System.Globalization.CultureInfo.CurrentCulture) 

                || x.StartsWith("b", true, System.Globalization.CultureInfo.CurrentCulture))

            .SelectMany(x => x)

            .ToArray();

        Console.WriteLine(resultTwo);                       

    }

}