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Math·Geometry and Trigonometry·Circles

hard

A circle has diameters $\overline{AC}$ and $\overline{BD}$. The circumference of the circle is $84\pi$, and the length of arc $DA$ is 2 times the length of arc $AB$. What is the length of arc $BC$?

A

$2\pi$

B

$14\pi$

C

$21\pi$

D

$28\pi$