A compound is drawn as a Fischer projection with vertical chain C1 ($\mathrm{CH}{}_3$, top), C2, C3, C4 ($\mathrm{CH}{}_3$, bottom): C2 has left H and right Br; C3 has left H and right Cl. This is subjected to these steps in order: (i) Br and H on C2 are interchanged; (ii) the whole projection is rotated by $180^{\circ }$ in the plane of the paper; (iii) Cl and H on C3 are interchanged. (C2 always denotes the carbon bearing Br.) What are the descriptors of the final compound and its relation to the starting compound? (Priorities: Br above C, Cl above C.)

1. A(2R,3R); diastereomer of the starting compound
2. B(2S,3R); identical to the starting compound
3. C(2R,3S); enantiomer of the starting compound
4. D(2S,3S); diastereomer of the starting compound