Let g(x)=∫2xf(t)dt, where f is continuous on an open interval (a,b). Which of the following conditions on f is sufficient to conclude that g is decreasing on (a,b)?
a) f(x)>0 for every x in (a,b)
b) f(x)≤0 for every x in (a,b)
c) f(x)<0 for every x in (a,b)
d) f(x)<0 for at least one x in (a,b)