Which of the following statements correctly describes the behavior of a differentiable function f on an interval where f′(x)>0 and f′′(x)<0 for every x in the interval?
a) The function is increasing and concave up on the interval.
b) The function is decreasing and concave down on the interval.
c) The function is increasing and concave down on the interval.
d) The function is decreasing and concave up on the interval.