3.35 There are two equally long roads from place A to place B; one is on flat ground, and the other has $\frac{1}{3}$ of the road uphill and $\frac{2}{3}$ of the road downhill. If the uphill speed is $\frac{1}{2}$ of the flat ground speed, and the flat ground speed is $\frac{1}{2}$ of the downhill speed, then from place A to place B
(A) the mountain road is faster.
(B) the flat road is faster.
(C) the mountain road and the flat road are equally fast.
(D) if the distance from A to B is long, then the mountain road is faster; if the distance is short, then the flat road is faster.
(E) if the distance from A to B is long, then the flat road is faster; if the distance is short, then the mountain road is faster.
(China Junior High School Mathematics Correspondence Competition, 1985)
[Solution] Let $S_{\text {flat }}, S_{\text {up }}, S_{\text {down }}$ represent the distances on flat ground, uphill, and downhill, respectively. According to the problem, $S_{\text {flat }} = S$, then $S_{\text {up }} = \frac{1}{3} S, S_{\text {down }} = \frac{2}{3} S$.
And.
$$
t_{\text {up }} = \frac{S_{\text {up }}}{V_{\text {up. }}}, \quad t_{\text {down}} = \frac{S_{\text {down }}}{V_{\text {down}}}, \quad t_{\text {flat }} = \frac{S}{V_{\text {flat }}}
$$

Also, by the problem statement,

Then we have

And
$$
V_{\text {up. }} = \frac{1}{2} V_{\text {flat }}, V_{\text {down }} = 2 V_{\text {flat }},
$$

Therefore,
$$
t_{\text {up }} = \frac{\frac{1}{3} S}{\frac{1}{2} V_{\text {flat }}} = \frac{2}{3} t_{\text {flat, }},
$$

Hence, the answer is $(C)$.