If a differentiable function f has a first derivative f′ that is increasing on the open interval (2,5), which of the following statements must be true about the graph of f on that interval?
a) f is increasing on (2,5).
b) The derivative f′ is positive on (2,5).
c) The graph of f is concave up on (2,5).
d) The graph of f is concave down on (2,5).