Answer: B
Explanation:
A is incorrect because going from -10 to 10 is not 0, it is 20.
C is incorrect because 21 km from sea level is adding 21.
D is incorrect because the total distance biking to school and back is actually 6.2.
B is correct because buying an item for the same price then selling it for the same price results in no increase in profit but consistency therefore being 0.