Determine the Degree of the Polynomial $- a + 9 a ^ { 3 } - 4 a ^ { 7 } + 4 a ^ { 2 }$

Determine the degree of the polynomial.
- $- a + 9 a ^ { 3 } - 4 a ^ { 7 } + 4 a ^ { 2 }$

A) 12
B) 7
C) 4
D) 13