Find the Average Value of the Function $f ( x ) = \sqrt { 4 - x ^ { 2 } }$

Multiple Choice

Find the average value of the function $f ( x ) = \sqrt { 4 - x ^ { 2 } }$ on the interval $[ - 2,2 ]$ .

A) $\frac { \pi } { 2 }$
B) $\pi$
C) 8 $\pi$
D) 3 $\pi$
E) 2 $\pi$
F) 4 $\pi$
G) 16 $\pi$
H) 6 $\pi$