In the following schematic diagram of a meiotic bivalent in diplotene, indicate whether each of the following chromatid pairs have undergone DNA exchange (E) or not (N). The different color of the final form of chromatid 3 is for clarity only. Your answer would be a four-letter string composed of letters E and N only, e.g. NENE.

( ) Chromatids 1 and 3
( ) Chromatids 1 and 4
( ) Chromatids 2 and 3
( ) Chromatids 2 and 4