How do you explain the fact that the various nomadic empires, particularly the mongols, were relatively short-lived

There were many reasons, most coming from the fact that the alliance among the tribes was not as strong as they only followed the leader that united them. After a leader would die the tribes would often each go their own way. They also had various religions and had no sense of national identity, so following a leader cult eventually had to be their end.

The answer to the question is a.