Answer: 2.
- A 2
- B 1
- C 0
- D 3
Correct answer: A. 2
Explanation: An ellipse has two foci; the sum of distances from any point to them is constant.
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.