Final answer:
The question seeks to identify the 2:1 ratio among parts of a circle, but none of the provided options inherently have a 2:1 ratio. The circumference to diameter ratio is π:1, which is approximately 3:1. The correct ratio between the radius and diameter is 1:2.
Step-by-step explanation:
The question asks which ratio corresponds to a 2:1 relationship in the context of a circle with point A being the center of the circle. Given the options provided, the correct answer is the ratio of the circumference to its diameter in a circle, which is universally known to be π:1. Since π is approximately 3.14159, it is roughly 3 times the length of the diameter, denoting a 3:1 ratio, not 2:1 as mentioned. None of the other options provided inherently have a 2:1 ratio.
Option A (Radius, Diameter) has a 1:2 ratio since the diameter is twice the length of the radius. Option B (Chord, Arc) does not have a fixed ratio, as it depends on the position of the chord and the arc. Option C (Circumference, Diameter) is a 3:1 ratio as mentioned earlier. Option D (Sector, Radius) does not have a 2:1 ratio because sectors come in various sizes, and their area is not dependent on the radius in a linear 2:1 ratio.