A sphere $O$ has radius 1. A pyramid has its apex at $O$ and the four vertices of its base all on the surface of sphere $O$. When the volume of this pyramid is maximized, its height is
A. $\frac { 1 } { 3 }$
B. $\frac { 1 } { 2 }$
C. $\frac { \sqrt { 3 } } { 3 }$
D. $\frac { \sqrt { 2 } } { 2 }$