Add the Radical Expressions $9 \sqrt { 10 } + 6 \sqrt { 10 }$

Multiple Choice

Add the radical expressions.
- $9 \sqrt { 10 } + 6 \sqrt { 10 }$

A) $15 \sqrt { 20 }$
B) $54 \sqrt { 10 }$
C) $15 \sqrt { 10 }$
D) Cannot be simplified further