Chapter 13 - Conic Sections - 13.2 Conic Sections: Ellipses - 13.2 Exercise Set - Page 861: 3

B is the center.

Ellipse are the set of all points in a plane for which the sum of the distance from two fixed points ${{F}_{1}}$ and ${{F}_{2}}$ is constant. The point ${{F}_{1}}$ and ${{F}_{2}}$ are called foci, the plural of focus. Thus, the blank can be filled by B.