What is behind the movement of blacks from rural to urban areas

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

i belive the continental association was created to boycott all contact with british goods.

i also belive it was in december 1, 1774