The answer key says b but i don't understand why explai

So that is the area under the curve from x=0 to x=4
when the line is below the curve, it is negative
from x=0, to x=2, that is area of 6
from x=2 to x=4, that is area of 6, but it is below the line so -6

6+-6=0

answer is B because that's how integrals work