Let f be a positive, increasing, differentiable function on (0,∞). Define G(x)=∫x2xf(t)dt for x>0. Which of the following must be true for all x>0?
a) G′(x)=0
b) G′(x)<0
c) G′(x)>0
d) The sign of G′(x) cannot be determined from the given information.