Answer: Choice B
Step-by-step explanation:
Usually (not always) the phrasing "the number of" refers to a whole number.
For example: the number of people in line buying a movie ticket. That could be say 10 people. We can't have 10.762 people since it makes no sense to have a fraction of a person.
Based on this alone, choices A, C and D are talking about whole numbers. In turn, this means these answer choices are discrete variables. It doesn't make sense to find the midpoint of 1 and 2 to have 1.5 of a song.
We can eliminate choices A, C and D.
Choice B on the other hand is a continuous variable. We can find the midpoint of any two distances to get some other distance. More importantly, this midpoint makes sense and is practical. Of course at some point, the measuring tools reach their limit of accuracy; however, I'm talking about theoretical distances.