Use Spherical Coordinates to Find the Z Coordinate of the Center

Multiple Choice

Use spherical coordinates to find the z coordinate of the center of mass of the solid lying between two concentric hemispheres of radii 4 and 7, and having uniform density k.

A) $\bar { z } = \frac { \pi } { 715 }$
B) $\bar { z } = \frac { 715 } { 248 } \pi$
C) $\bar { z } = 248$
D) $\bar { z } = \frac { 429 } { 149 }$
E) $\bar { z } = \frac { 715 } { 248 }$