How was imperialism driven by economic necessity?

Economic expansion demanded cheap labor, access to or control of markets to sell or buy products, and natural resources such as precious metals and land; governments have met these demands by hook (tribute) or by crook (plunder).

Explanation:

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