new round of warfare between the Sioux and U. S. Army began in 1874 when a Colonel George Custer led an expedition to Little Big Horn, Montana. b Sioux Chief Crazy Horse began an effort to drive all whites from Montana and the Dakotas. c Colonel George Custer discovered gold on Sioux land in the Black Hills. d the federal government announced that it was opening all Sioux lands to settlement. e the U. S. Army decided to retaliate for the Fetterman massacre.

c. Colonel George Custer discovered gold on Sioux land in the Black Hills.

Explanation:

In 1876 Colonel George Custer discovered the existence of gold on the Back Hills property, which belonged to the Sioux, an allied North American Indian tribe. The discovery of this gold attracted the attention of many settlers who began to invade these lands and strongly displeased both the Sioux and the Chayennne who were their allies. Both tribes refused to accept settler entry into their properties, but the greed of the settlers and the government was not discouraged and sparked a new round of war between the Sioux and the US Army.

these conditions long with a short growing season made farming difficult.

explanation:

the north has a climate of warm summers and snowy cold winters. the terrain is rocky, hilly, and not good for farming.