Let H(x)=∫x23x1+t4dt for x>0. Which of the following describes the sign of H′(x) for x>0?
a) H′(x) is positive for all x>0.
b) H′(x) is negative for all x>0.
c) The sign of H′(x) varies depending on the value of x.
d) H′(x) is positive when x>1 and negative when 0<x<1.