Which statement correctly relates the slope of the graph of the first derivative f′ to the value of the second derivative f′′ at a point where both derivatives exist?
a) If the slope of f′ is positive at the point, then f′′(x)<0.
b) If the slope of f′ is zero at the point, then f′′(x) must be negative.
c) If the slope of f′ is positive at the point, then f′′(x)>0.
d) If the slope of f′ is negative at the point, then f′′(x)>0.