A because if he started at negative ten you would add the 10 he increased. When you add those two together you get 0.
-10+10=0
Odd question; there's no real rate exact 50 meters per lap (assuming the last is a typo), i.e we have a 50 meter pool.
d = 50 n
so when n=15
d = 50(15) = 750 meters
The answer is false sorry it is true no it is false sorry don,t know