Answer: focus.
- A focus
- B vertex only
- C centre
- D radius
Correct answer: A. focus
Explanation: A parabola is the locus of points equidistant from the focus and the directrix.
All four conics form a single family distinguished only by eccentricity: a circle is the most "closed" (e=0), an ellipse is an elongated closed curve, a parabola is the borderline open curve (e=1), and a hyperbola has two separate open branches (e>1).
Concept context
Study circles, parabolas, ellipses, and hyperbolas as curves formed by intersecting a plane with a double cone, with their standard equations and key properties.