Which of the following operations on a Fischer projection leave the configuration of every stereocentre unchanged?

Arotating the whole projection by $18 0^{\circ}$ in the plane of the paper
Brotating the whole projection by $9 0^{\circ}$ in the plane of the paper
Cinterchanging two groups on one stereocentre twice in succession
Dinterchanging the groups at one stereocentre just once

Answer: A, C

18 0^{\circ}

in-plane rotation preserves the vertical-back, horizontal-forward convention, so nothing changes. Two successive interchanges at the same centre restore the original arrangement. A

9 0^{\circ}

rotation or a single interchange inverts the configuration, so these are not correct.

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