3. $\begin{aligned} \mathrm{C} \quad \frac{1}{1-t} A(t) & =\left(1+t+t^{2}+t^{3}+\cdots\right)\left(a_{0}+a_{1} t+a_{2} t^{2}+\cdots\right) \\ & =a_{0}+\left(a_{0}+a_{1}\right) t+\left(a_{0}+a_{1}+a_{2}\right) t^{2}+\cdots .\end{aligned}$
3. Suppose the ordinary generating function of the sequence $\left\{a_{0}, a_{1}, a_{2}, \cdots\right\}$ is $A(t)$, then the ordinary generating function of the sequence $\left\{a_{0}, a_{0}+a_{1}, a_{0}+a_{1}+\right.$ $\left.a_{2}, \cdots\right\}$ is
A. $(1-t) A(t)$
B. $(1+t) A(t)$
C. $\frac{1}{1-t} A(t)$
D. $\frac{1}{1+t} A(t)$