Q5

If $\displaystyle\int_a^x f(t)\,dt=g(x)$, which of the following is a primitive of $f(x)g(x)$?

A. $\frac12[f(x)]^2$

B. $\frac12[f'(x)]^2$

C. $\frac12[g(x)]^2$

D. $\frac12[g'(x)]^2$