Steel ball B -
Constant acceleration when falling in air.
Then hits oil - suddenly there is a lot of drag because the ball is moving quite quickly and the oil is thick.
This means there is a resultant force upwards - so the ball will decelerate.
As it slows down the drag force also decreases until it balances the weight, there is then no resultant force so no acceleration.
When it hits the bottom there will be a sudden deceleration.