I do not have your choices of "the following" Here are some reasons I learned: Christianity, empire was too big and became hard to manage, there were many nonromans attacking parts of empire- Goths, the emperors themselves were weak or insane intrigue was rampant.
It's C. Germany sunk the Lusitania and tried to get Mexico to attack the US in the Zimmerman Telegram.
Yes more than likely they were I'm not 100% sure though
