Simplify the Expression $6 ^ { 4 } \cdot 6 ^ { 7 }$

Simplify the expression. Write your answer with positive exponents only. $6 ^ { 4 } \cdot 6 ^ { 7 }$

A) $6 ^ { - 3 }$
B) $36 ^ { 11 }$
C) $6 ^ { 28 }$
D) $6 ^ { 11 }$