If f′(x)>0 and f′′(x)<0 for every x in the open interval (1,4), which of the following best describes the graph of f on that interval?
a) It is increasing and concave up on (1,4).
b) It is decreasing and concave down on (1,4).
c) It is increasing and concave down on (1,4).
d) It is decreasing and concave up on (1,4).