When is it correct to use "welcher" as a relative pronoun, and when should we use "der" instead?
For instance, should I write:
Schreiben Sie ein Programm, welches die Lösung findet.
or
Schreiben Sie ein Programm, das die Lösung findet.
The second seems better, but it's not clear to me, since "Programm" is indefinite. Does this make a difference in the choice?