Hi:
Here is a link to MSDN documentation on joining strings in a string array with specified separator (String.Join method). Then, instead of using WriteAllLines, you could use the File.WriteAllText method?
So maybe something like:
string [] lines = { A1, B1, C1};
string allLines = String.Join(",", lines, 0, 3);
System.IO.File.WriteAllText(@"C:\Users\Lawson\Documents\output.csv", allLines);
So you know, I haven't tried to compile or run that, just typed it in, so it could have an error or two, but just wanted to give you the gist...