The answer is A
In the depletion layer, electrons combine with holes producing charge density so that region is depleted of charge carriers but it has charge density .
Conclusion: Neutral but still having non zero charge density .
This picture develops a clear understanding.
In the air, there's nothing holding the body of the helicopter still - so the rotor blades spin one direction, but the torque applied to spin the rotors tries to spin the body of the helicopter in the opposite direction. The little rotor on the tail helps keep the body of the helicopter pointed in the same direction.
Balance on a bicycle is a matter of constantly correcting against falls, and it's easier when the speed is higher because the inertia of moving forward overcomes the need for corrective actions.
