What describes θ when an ellipse is created from an intersection of a plane and double-napped cone?

For an ellipse, the cutting plane must intersect only one nappe of the cone and must not be parallel to a generator. If θ is the angle between the plane and the cone’s axis, this means it lies between the cone’s half-angle α and 90°: α < θ < 90°. Thus, the correct choice is “α < θ < 90°.”

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