Definition: In this task, you will be presented with a passage and have to answer a question based on your understanding of the events inferred from the passage. Try to find the best answer that is most likely to fill in "_". Note that the URLs in the text have been replaced with [Link].
Input: (CNN) The Sudanese government has been accused of using chemical weapons against the people of Darfur, according to a report released by Amnesty International on Thursday. The attacks targeted civilians and may constitute a war crime, the report said. The Sudanese government has denied the allegations, calling them "rumors." Amnesty says it has new evidence abuses persist in a war that has been described as one of the world's worst humanitarian conflicts by the United Nations. Unidentified witnesses quoted in the Amnesty report said the attacks left a smoke, which turned dark blue and smelled "like rotten eggs," coating the trees, ground, and humans in a thick black dust.Chemical weapons deliberately against civilians in Darfur conflict, says AmnestyThe attacks may constitute a war crime, the report said.Witnesses say skin became rotten and fell off in chunks after attacks. 
 Question:The conflict in Darfur began around 2003 when several rebel groups in _ took up arms against the government in Khartoum.

Darfur