Answer:
(C) The graph is continuous because there can be fractional values for time.
Explanation:
There can't be a negative altitude because when it reaches the ground, it has a altitude of zero and can't go further down. However, it's not B because a discrete graph would also mean there would be set times (represented by points on the graph) of the hot air balloon moving. This isn't true, because the hot air ballon moves as time continues, even fractional time values.
So it's C, "The graph is continuous because there can be fractinoal values for time".
Hope it helps (●'◡'●)