$26 \cdot 2$ Given that the curve in the figure is part of a circle centered at the origin with a radius of 1, the equation of this curve is
(A) $\left(x+\sqrt{1-y^{2}}\right)\left(y+\sqrt{1-x^{2}}\right)=0$.
(B) $\left(x-\sqrt{1-y^{2}}\right)\left(y-\sqrt{1-x^{2}}\right)=0$.
(C) $\left(x+\sqrt{1-y^{2}}\right)\left(y-\sqrt{1-x^{2}}\right)=0$.
(D) $\left(x-\sqrt{1-y^{2}}\right)\left(y+\sqrt{1-x^{2}}\right)=0$.
[Solution] $\because \because$ The curve is the union of the right half of the unit circle and the lower half of the unit circle. Note that
the equation of the right half of the unit circle is $x-\sqrt{1-y^{2}}=0$,
the equation of the lower half of the unit circle is $y+\sqrt{1-x^{2}}=0$.
Thus, the equation of the given curve is $\left(x-\sqrt{1-y^{2}}\right)\left(y+\sqrt{1-x^{2}}\right)=0$. Therefore, the answer is (D).