To lower or to upper

Given a string which has at least two words separated by space, write a method that changes first word in the string to upper case, second to lower case and so on.
Expected input and output
ToLowerOrToUpper("this is it") → "THIS is IT"
using System;

namespace CSharpExercises.Exercises.Library_functions
{
    class ToLowerOrToUpperTask
    {
        public static string ToLowerOrToUpper(string word)
        {
            string[] words = word.Split(' ');

            for (int i = 0; i < words.Length; i++)
            {
                words[i] = i % 2 == 0 ? words[i].ToUpper() : words[i].ToLower();
            }

            return String.Join(" ", words);
        }

        public static void Main()
        {
            Console.WriteLine(ToLowerOrToUpper("aaa BBB ccc DDD"));                             // AAA bbb CCC ddd
            Console.WriteLine(ToLowerOrToUpper("Etiam mollis lectus ac facilisis venenatis"));  // ETIAM mollis LECTUS ac FACILISIS venenatis
            Console.WriteLine(ToLowerOrToUpper("th1s 15 5amp13 53nt3nc3"));                     // TH1S 15 5AMP13 53nt3nc3
        }
    }
}