A differentiable function f is positive and strictly decreasing on the interval (1,4). For the accumulation function g(x)=∫2xf(t)dt, which statement about the graph of g on (1,4) is most accurate?
a) g is increasing and concave up on (1,4).
b) g is decreasing and concave up on (1,4).
c) g is increasing and concave down on (1,4).
d) g is decreasing and concave down on (1,4).