Help with string array

Hi,
I have a string that I have split into an array as follows;
string str = "one two three four";
            string[] arr = str.Split(' ');
            foreach(string item in arr)
                Console.WriteLine(item);
What I need to do is output another string which is like;
"one and two, two and three, three and four", so basically need to read each item and append the word and in between each one, this may be an obvious answer but I am fairly new to .NET so would appeciate some help.
Note, that I dont want one and two and three and four, I need "one and two, two and three, three and four"
Cheers
Chris



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