C# Why are some of these methods using delegates and generics not starting?

I was trying to learn a bit about generics and the SelectMany function and delegates. The semantic function of the code below(as in what the functions are supposed to be doing), doesn't matter. Most of them do nothing meaningful and are just attempts to see if the issue of the function not starting, persists.

That's my question.. Why is it that the method named _SelectManyIteratora doesn't start. Nor does _SelectManyIteratorb. But _SelectManyIteratorc does start.

As in, i'm calling the function, i'm not getting any compilation error, and the first line of the function, a simple WriteLine statement, is not executing!

I would have expected them all to start.

My interest is

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

using System.Collections;


namespace functionwontstart
{
    class Program
    {
        static void Main(string[] args)
        {
            string[] arrstr1 = { "abc", "def", "ghi" };
            string[] arrstr2= {"qrs","tuv","wxy"};

            _SelectManyIteratora(arrstr1, x => arrstr2, (s1, s2) => s1 + s2);
            _SelectManyIteratorb(arrstr1, x => arrstr2, (s1, s2) => s1 + s2);
           _SelectManyIteratorc(arrstr1, x => arrstr2);
            Console.ReadLine();
        }


        static IEnumerable<TResult> _SelectManyIteratora<TSource, TCollection, TResult>(IEnumerable<TSource> source, Func<TSource, IEnumerable<TCollection>> collectionSelector, Func<TSource, TCollection, TResult> resultSelector)
        {
            Console.Write("got this far"); // no!

            foreach (TSource element in source)
                foreach (TCollection subElement in collectionSelector(element))
                    yield return resultSelector(element, subElement);                
        }

        static IEnumerable<TResult> _SelectManyIteratorb<TSource, TCollection, TResult>(IEnumerable<TSource> source, Func<TSource, IEnumerable<TCollection>> collectionSelector, Func<TSource, TCollection, TResult> resultSelector)
        {
            Console.Write("got this far"); // no!

            TSource ts = source.ElementAt(0);
            TCollection tc = collectionSelector(ts).ElementAt(0);
            yield return resultSelector(ts, tc);
        }

        static IEnumerable<int> _SelectManyIteratorc<TSource, TCollection>(IEnumerable<TSource> source, Func<TSource, IEnumerable<TCollection>> collectionSelector)
        {
            Console.Write("got this far"); // yes

            return new List<int>();
        }

      } //class



} //namespace