4. [1 points] Consider a Helium balloon freely rising under gravity in air. Choose all true statements. (a) The net force acting on the balloon is zero at all times. (b) Even though gravity is pulling the balloon down, the net acceleration of the balloon is zero after it reaches terminal speed. (c) As the balloon is still rising, all forces acting on it are not balanced after reaching the terminal speed. Answer: The true statement(s) is (are):

Fig: 1