Which idea is left unexplained in the text? where and how Rivera became an expert on Mexico’s past where and how Rivera became an expert on Mexico’s past the scale and content of Rivera’s epic work, "The History of Mexico" the scale and content of Rivera’s epic work, "The History of Mexico" why Rivera’s membership in the Communist Party was controversial why Rivera’s membership in the Communist Party was controversial what Rivera gained from the time he spent living in Paris what Rivera gained from the time he spent living in Paris