Which of the following is not a correct procedure?

Which of the following is not a correct procedure?

A.Controllers should pass a clearance slowly and clearly to avoid wasteful repetition

B.A route clearance should be passed to an aircraft before start up

C.Controllers should avoid issuing a clearance to a pilot occupied in taxiing maneuvers

D.On specific occasions, a clearance may be passed to a pilot engaged in line-up or takeoff maneuvers

正确答案是D