What does the phrase "a grave injustice " mean? ?

Read this excerpt from "we shall overcome." hundreds of thousands of people sang them. but sometimes, it was one lone person with a guitar. they sang in segregated bus stations, picket lines, freedom marches, concert stadiums, city squares, and videos. when injustice of oppression threatens, people sing protest songs to proclaim their resistance, publicize their cause, and encourage hope for a better future, based on this excerpt as an introductory paragraph, which of the following would you expect to read about in the article? different groups of people who were oppressed the ways that music brings diverse groups of people together similarities between historical movements that provided rights for groups of people different song styles throughout specific historical events