Let f be differentiable on an interval I, and define the accumulation function F(x)=∫0xf(t)dt. Which of the following statements correctly describes where F is concave up?
a) F is concave up where f′′(x)>0.
b) F is concave up where f′(x)>0.
c) F is concave up where f(x)>0.
d) F is concave up where f′(x)<0.