I think it is d if I'm wrong I'm sorry
I can't help you with your second question but heres a site that I've found to help you with your 1st one. I hope this helps^-^
https://nationalinterest.org/feature/5-big-lessons-the-first-gulf-war-13536
Answer:
Invasions by Barbarian tribes
Western Rome's collapse pins the fall on a string of military losses sustained against outside forces. Rome had tangled with Germanic tribes for centuries, but by the 300s.