Solve the Problem $z = 9 - x ^ { 2 } - y ^ { 2 }$

Multiple Choice

Solve the problem.
-Find the average height of the part of the paraboloid $z = 9 - x ^ { 2 } - y ^ { 2 }$ that lies above the $x y$ -plane.

A) 3
B) $\frac { 9 } { 2 }$
C) $\frac { 27 } { 2 }$
D) $\frac { 45 } { 2 }$