A Little Girl Sits in a Car at a Traffic

Multiple Choice

A little girl sits in a car at a traffic light holding a helium-filled balloon as shown in the illustration below. The windows are closed and the car is relatively airtight. When the light turns green and the car accelerates forward, her head pitches backward but the balloon pitches forward. Why?

A) This demonstration will only work with a helium filled mylar balloon since the mylar is unaffected by the motion of the air moving backwards inside the car.
B) Since helium is lighter than air, it will follow the path of the car and not the direction of movement of the air within it.
C) Since the car is relatively airtight and the balloon is not anchored to the car, the balloon will be unaffected by the acceleration of the car.
D) The air inside the car has more inertia than the helium in the helium balloon. Therefore, although the air pitches backwards, the lighter helium moves forward.