A. Accelerate to make a gap behind you
B. Touch the brakes sharply to show your brake lights
C. Maintain your speed to prevent the vehicle from overtaking
D. Allow the vehicle to overtake
Try the question by clicking here
Don't enforce the speed limit by blocking another vehicle's progress. This will only lead to the other driver becoming more frustrated. Allow the other vehicle to pass when you can do so safely.
Technically you should only flash your headlights to warn others of your presence. Don't use them to greet others, show impatience or give priority to other road users. They could misunderstand your signal. However, sometimes people have an emergency for one reason or another.