$\text{A.}$ $\cos \frac{\frac{\pi}{2}+2 k \pi}{4}+\mathrm{i} \sin \frac{\frac{\pi}{2}+2 k \pi}{4}(k=0,1,2,3)$
$\text{B.}$ $\sin \frac{\frac{\pi}{2}+2 k \pi}{4}+\mathrm{i} \cos \frac{\frac{\pi}{2}+2 k \pi}{4}(k=0,1,2,3)$
$\text{C.}$ $\cos \frac{\pi}{8}+\mathrm{i} \sin \frac{\pi}{8}$
$\text{D.}$ $\cos \frac{\frac{\pi}{2}+k \pi}{4}+\mathrm{i} \sin \frac{\frac{\pi}{2}+k \pi}{4}(k=0,1,2,3)$
$\text{E.}$
$\text{F.}$