What is the observational evidence that black hole jets are launched very close to the event horizon?

Question

Quizplus · Accepted Answer

The relativistic speeds of the jet are the strongest observational evidence that black hole jets are launched very close to the event horizon. These speeds often approach the speed of light, indicating that the jet must be launched from very close to the black hole where the intense gravitational forces accelerate the material to high velocities. The other choices, such as the narrowness of the jet, gravitational lensing by the black hole, and imaging of the accretion disk, are related to black hole jets but do not necessarily indicate their launch point.

What Is the Observational Evidence That Black Hole Jets Are