A 24 g, 3-cm-diameter thin, hollow sphere rotates at 30 rpm about a vertical, frictionless axis through its center. A 4 g bug stands at the top of the sphere. He then walks along the surface of the sphere until he reaches its "equator." When he reaches the equator, the sphere is rotating at 

A. 15 rpm 

B. 24 rpm 

C. 30 rpm 

D. 37 rpm 

E. 45 rpm