A Sphere Is Dropped from a Great Height and When

Multiple Choice

A sphere is dropped from a great height and when its speed reaches 20 m/s,its acceleration downward is 8.4 m/s².If the drag force is proportional to the square of the speed,what will be its acceleration downward when its speed reached 40 m/s?

A) 2.6 m/s²
B) 4.2 m/s²
C) 8.0 m/s²
D) 0 m/s² because it has now reached terminal velocity.