What
factors led to the fall of rome?

Rome might have fallen because of Christianity. They had many foreign enemies that they couldn't fight against and win. The last reason is barbarian invasions.

Hope this helps.

the answer to your question is a. the salt plains

in 1811, he led an expedition, known as the george c. sibley expedition, to improve relations with the pawnee and kansa tribes, and also to locate the rumored jefferson's salt mountain