Solve the Equation by Making an Appropriate Substitution $\mathrm { x } ^ { 1 / 2 } - 10 \mathrm { x } ^ { 1 / 4 } + 21 = 0$

Multiple Choice

Solve the equation by making an appropriate substitution.
- $\mathrm { x } ^ { 1 / 2 } - 10 \mathrm { x } ^ { 1 / 4 } + 21 = 0$

A) $\{ 81,2401 \}$
B) $\{ 9,49 \}$
C) $\{ - 3 , - 7 \}$
D) $\{ 3,7 \}$