Which of the following correctly relates the sign of f′′(x) to the concavity of a twice-differentiable function f on an interval I?
a) If f′′(x)>0 for all x∈I, then f is concave down on I.
b) If f′′(x)>0 for all x∈I, then f is increasing on I.
c) If f′′(x)>0 for all x∈I, then f is concave up on I.
d) If f′′(x)<0 for all x∈I, then f is concave up on I.