Let the distance from O to the straight line be d, then 1-d under AB=2? , that is, s △ AOB = D. 1-d under the root sign? , so s =d? ( 1-d？ ) ≤ 1/4, only if d? = 1-d？ The equal sign holds, and d= root sign 2/2.

That is, d= (formula of distance from point to straight line) = root number 2/2, and the solution is K = root number 3/3. Because the curve is a semicircle, we can only take k=- root 3/3 and choose B.