30. A helicopter on a landing pad weighs 2000 newtons. As the helicopter blades begin to turn, they push air downward with increasing force. When will the helicopter lift off the ground?

a. When the force of the downward air is greater than the helicopter’s weight.
b. When the force of the downward air is exactly twice the helicopter’s weight.
c. When the force of the downward air is exactly half of the helicopter’s weight.
d. When the force of the downward air is exactly equal to the helicopter’s weight.