Simplify by First Converting to Rational Exponents $\sqrt[3]{\mathrm{r}} \cdot \sqrt[7]{\mathrm{r}^{3}}$ A) $\sqrt[21]{\mathrm{r}^{4}}$

Multiple Choice

Simplify by first converting to rational exponents. Assume that all variables represent positive real numbers.
- $\sqrt[3]{\mathrm{r}} \cdot \sqrt[7]{\mathrm{r}^{3}}$

A) $\sqrt[21]{\mathrm{r}^{4}}$
B) $\sqrt[21]{\mathrm{r}^{16}}$
C) $\sqrt[10]{\mathrm{r}^{3}}$
D) $\sqrt[10]{r^{4}}$