The American attitude shifted in favor of rights for Native Americans. This is based on "Polls showed that Americans believed in rights for Native Americans". Why it's not B is because it wasn't the polling that contributed to the act, but what the polls showed which is C.
In subsistence farming, planting decisions are made based on a family's needs, whereas in cash-crop farming, farmers plant strategically to capitalize on demand and market prices. While some farmers grow crops for the sole purpose of feeding their families, others engage in both cash crop and subsistence farming