The CORRECT answer is:
The distance on the arc between the two points is twice the radius of the circle.
Explanation:
Radius times Radius = Diameter (180 times 180 = 360)
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A whole circle would be 360°, but since it is splitting directly in half of the circle and is looking for half the distance, it would be 180°.
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To be sure:

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If you was to go even further, such as distance for A to D, it would be 1/4 of 360 (1/4 of the circle) which would be 90° since half of 180 is 90. The same will go for the distance from B to D, it would also become 90°.
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Thinking outside the "circle" (**BOX), C to D will also be 90°. You can also see it if you connect A to C, and C to D which will make it a 90° angle.
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Overall, this kind of math is kind of like cutting pieces of pie with your friends. This math can be done easily, but can also lead to probabilities and circumference of a circle.
©CDJ