Add or Subtract $\sqrt { 48 } - \sqrt { 147 }$

Multiple Choice

Add or subtract. Assume all variables represent positive real numbers.
- $\sqrt { 48 } - \sqrt { 147 }$

A) $- 3 \sqrt { 6 }$
B) $11 \sqrt { 3 }$
C) $- 9$
D) $- 3 \sqrt { 3 }$