Which explains why supreme court justices are given lifetime appointments?

I believe the answer is: They protect justices from political pressure.

If supreme court is replaced every time the president is changed, the chosen supreme court would most likely feel pressured to conform to the will of the political parties that help his/her election. To prevent this, the supreme court is appointed for lifetime.

answer; the period of time known as the middle ages;

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