L-Alanine is written as a Fischer projection with $\mathrm{COOH}$ at the top, $\mathrm{CH}{}_3$ at the bottom, and on the central carbon $\mathrm{NH}{}_2$ on the left and H on the right. Its CIP descriptor is:

The priorities are $\mathrm{NH}{}_2$, then $\mathrm{COOH}$, then $\mathrm{CH}{}_3$, then H. H is on a horizontal bond. Going from $\mathrm{NH}{}_2$ (left) to $\mathrm{COOH}$ (top) to $\mathrm{CH}{}_3$ (bottom) is clockwise, which must be reversed because H is horizontal, so the centre is S. D and L are not CIP descriptors.