Add or Subtract $$12 \sqrt [ 3 ] { 3 } + 9 \sqrt [ 3 ] { 3 }$$

Multiple Choice

Add or subtract. Assume all variables represent positive real numbers.
- $$12 \sqrt [ 3 ] { 3 } + 9 \sqrt [ 3 ] { 3 }$$

A) $21 \sqrt [ 3 ] { 6 }$
B) $- 3 \sqrt [ 3 ] { 3 }$
C) $21 \sqrt [ 3 ] { 3 }$
D) $21 \sqrt [ 3 ] { 9 }$