The Numerical Aperture of a Fiber

The numerical aperture of a fiber:

A) is a basic specification which indicates its ability to accept light and shows how much light can be off axis and still be propagated.
B) the acceptance angle where the output light is no greater than 10 db down from the peak value.
C) defines the half-angle of the cone of acceptance for propagated light in the fiber.
D) all of the above