A student blows a ball through a vertical tube as shown in the figure.
The ball speeds up the tube. This happens because
a. the weight of the ball changes when the student blows air through the tube.
b. the force exerted by the air blown by the student is equal to the weight of the ball.
c. the weight of the ball is greater than the force of the air blown by the student.
d. the force exerted by the air blown by the student is greater than the weight of the ball.