$\text { Find the critical number of the function } f ( x ) = - 6 x ^ { 2 } + 108 x + 7$

Multiple Choice

$\text { Find the critical number of the function } f ( x ) = - 6 x ^ { 2 } + 108 x + 7$

A) $x = 18$
B) $x = 9$
C) $x = 84$
D) $x = 14$
E) $x = 7$