How to explain the difference between main idea and central idea?

Read the text of the barack obama speech “remarks by the president at the 50th anniversary of the selma to montgomery marches.” which options best explain obama’s rhetorical purpose in the speech? (select all that apply.) `to celebrate an important milestone in the history of civil rights `to explain to his audience why they should welcome immigrants into america `to inspire and motivate his audience to exercise their right to vote `to inform and educate his audience about the history of social injustice in america