1. Simplify the trigonometric rational expression
$$
\frac{\cos ^{4} x+\sin ^{4} x+\sin ^{2} x \cdot \cos ^{2} x}{\sin ^{6} x+\cos ^{6} x+2 \sin ^{2} x \cdot \cos ^{2} x}
$$

The value is ( ).
(A) 1
(B) $\sin x+\cos x$
(C) $\sin x \cdot \cos x$
(D) $1+\sin x \cdot \cos x$
1. A.
$$
\begin{array}{l}
\sin ^{6} x+\cos ^{6} x+2 \sin ^{2} x \cdot \cos ^{2} x \\
=\left(\sin ^{2} x+\cos ^{2} x\right)\left(\sin ^{4} x+\cos ^{4} x-\right. \\
\left.\sin ^{2} x \cdot \cos ^{2} x\right)+2 \sin ^{2} x \cdot \cos ^{2} x \\
=\sin ^{4} x+\cos ^{4} x+\sin ^{2} x \cdot \cos ^{2} x
\end{array}
$$