(a) Partial dehydrohalogenation of either (1R, 2R)-1,2-dibromo-1,2-diphenylethane or (1S, 2S)-1,2-dibromo-1,2-diphenylethane enantiomers (or a race mate of the two) produces (Z)-1-bromo-1,2-diphenylethene as the product, whereas
(b) Partial dehydrohalogenation of (1R, 2S)-1, 2-dibromo-1, 2-diphenylethane (the meso compound) gives only (E)-1-bromo-1, 2-diphenylethene.
(c) Treating (1R, 2S)-1, 2-dibromo-1, 2-diphenylethane with sodium iodide in acetone produces only (E)-1,2-diphenylethene. Explain these results.