Let $R$ be the radius of the circle. What is the angle subtended by an arc of length $R$ at the center of the circle?
- A. 1 degree
- B. 1 radian
- C. 90 degrees
- D. $\pi$ radians
Answer: B. 1 radian
Explanation: By definition, 1 radian is the angle subtended by an arc equal in length to the radius.