Which statement is not true?

a. dr. martin luther king, jr. gave the "i have a dream" speech in 1963.
b. states now forbid operators of theaters, hotels, and restaurants to refuse service to minority groups.
c. slavery was an established institution until the twelfth amendment was ratified on december 18, 1865.
d. in 1870 the fifteenth amendment gave all male citizens the right to vote.