Example 4. (Shanghai Mathematics, 1987)
$$
\log _{3}\left[(3+1)\left(3^{2}+1\right)\left(3^{4}+1\right) \cdot \ldots\right.
$$
- $\left.\left(3^{84}+1\right)+\frac{1}{2}\right]+\log _{3} 2$ The value is ( ).
(A) 32.
( B) 64.
(C) 128.
(D) None of the above.
$$
\begin{aligned}
\text { Original expression }= & \log _{3}(3-1)(3+1)\left(3^{2}+1\right) \\
& \cdot \cdots \cdot\left(3^{84}+1\right) \\
= & \log _{3} 3^{128}=128 .
\end{aligned}
$$

Expressing "3-1" as 2 in the second line is a kind of simplification. This can be referred to as the method of adding factors.