1.99 If $x \geqslant 0$, then $\sqrt{x \sqrt{x \sqrt{x}}}$ equals
(A) $x \sqrt{x}$.
(B) $x \sqrt[4]{x}$.
(C) $\sqrt[8]{x}$
(D) $\sqrt[8]{x^{3}}$.
(E) $\sqrt[8]{x^{7}}$.
(42nd American High School Mathematics Examination, 1991)
[Solution] Original expression $=x^{\frac{1}{2}} \cdot x^{\frac{1}{4}} \cdot x^{\frac{1}{8}}=x^{\frac{7}{8}}=\sqrt[8]{x^{7}}$. Therefore, the answer is $(E)$.