\begin{gather*} r, s, t \end{gather*} In the sequence above, if each term after the first is x more than the previous term, what is the average of r, s, and t in terms of r and x?
(A)\(r + x\) (B)\(r + \frac{x}{3}\) (C)\(r + \frac{2x}{3}\) (D)\(\frac{r + x}{3}\) (E)\(3(r + x)\)
A