D(√3， 1)

Let AD tilt angle be α, BD tilt angle be β and AB tilt angle be π/6.

The angle between AB and AD is ∠ 1=α-π/6.

The included angle between AB and BD ∠2=π/6+π-β.

We can know ∠ 1=∠2 from the nature of the circle.

∴α-π/6=π/6+π-β==>； α+β=4π/3

Option c