Add or Subtract $\sqrt [ 3 ] { a } + \sqrt [ 3 ] { 8 a }$

Multiple Choice

Add or subtract. Assume all variables represent positive real numbers.
- $\sqrt [ 3 ] { a } + \sqrt [ 3 ] { 8 a }$

A) $4 \sqrt [ 3 ] { 8 a }$
B) $6 \sqrt [ 3 ] { a }$
C) $5 \sqrt [ 3 ] { a }$
D) $3 \sqrt [ 3 ] { a } + \sqrt [ 3 ] { 8 a }$