Simplify by First Converting to Rational Exponents $\sqrt[3]{x^{21}}$ A) $\frac{z}{3}$ B) $x^{63}$ C) $x^{7}$

Multiple Choice

Simplify by first converting to rational exponents. Assume that all variables represent positive real numbers.
- $\sqrt[3]{x^{21}}$

A) $\frac{z}{3}$
B) $x^{63}$
C) $x^{7}$
D) $3 z$