To prove that a differentiable function s is strictly decreasing on an interval (m,n), which additional hypothesis is sufficient?
a) s′(x)≤0 for all x in (m,n) and s′(x)=0 somewhere in (m,n).
b) s′(x)≥0 for all x in (m,n).
c) s′(x)<0 for all x in (m,n).
d) s′(x) changes sign exactly once on (m,n).