$20 \cdot 21$ In the unit circle, chords $P Q$ and $M N$ are parallel to the unit radius $O R$ through the center $O$. Chords $M P, P Q$, and $N R$ are all $s$ units long, and chord $M N$ is $d$ units long. Of the following three equations
$\mathrm{I} . d-s=1$;
II . $d s=1$;
III . $d^{2}-s^{2}=\sqrt{5}$.

which must be correct?
(A) Only I.
(B) Only II.
(C) Only III.
(D) Only I and II.
(E) I, II, and III.
(24th American High School Mathematics Examination, 1973)