C# - IEnumerable to delimited string

What is the functional programming approach to convert an IEnumerable<string> to a delimited string? I know I can use a loop, but I'm trying to wrap my head around functional programming.

Here's my example:

var selectedValues =
from ListItem item in checkboxList.Items
where item.Selected
select item.Value;

var delimitedString = ??

.. or could I do this in just the first var assignment (append each result to the previous)?