Consider the function [tex]f(x)=x^2+1[/tex], which is always positive. The derivative is
[tex]f'(x)=2x[/tex]
which is negative for [tex]x<0[/tex] and positive for [tex]x>0[/tex]. This example alone tells you that you can't generally describe the sign of the derivative of a strictly positive function, so the answer is E.
