Consider the following fill-in:
Mackenzie's clarinet squealed like a startled puppy, __ she hadn't practiced in weeks.
The presentation I'm looking at indicates "for" as the correct solution.
To me, "because" also sounds fine, but I would not use a comma after puppy in that case.
Is that essentially the reason "for" is best here, because of the way the fill-in is punctuated? Is "...puppy because..." valid also?