Fill in the Blank with One of the Words or Phrases

Multiple Choice

Fill in the blank with one of the words or phrases listed below. $$\begin{array} { l l l } \text { circle } & \text { ellipse } & \text { hyperbola } \\\text { conic sections } & \text { vertex } & \text { diameter } \\\text { center } & \text { radius } & \text { nonlinear system of equations }\end{array}$$
-Twice a circle's radius is its .

A) diameter
B) conic section
C) vertex
D) center