1. A certain road consists of three equal-length sections: uphill, flat, and downhill. It is known that a car travels at average speeds of $v_{1} 、 v_{2} 、 v_{3}$ on each section, respectively. Then the average speed of this car over the entire road is ( ).
(A) $\frac{v_{1}+v_{2}+v_{3}}{3}$
B) $\frac{\frac{1}{v_{1}}+\frac{1}{v_{2}}+\frac{1}{v_{3}}}{3}$
(C) $\frac{1}{\frac{1}{v_{1}}+\frac{1}{v_{2}}+\frac{1}{v_{3}}}$
(D) $\frac{3}{\frac{1}{v_{1}}+\frac{1}{v_{2}}+\frac{1}{v_{3}}}$
- 1.D.

Solution 1: Let the total length of the road be $3s$, then the lengths of the three different sections are all $s$. The time taken by the car to travel each section is $t_{i}=\frac{s}{v_{i}}(i=1,2,3)$, and the average speed of the car over the entire road is
$$
\bar{v}=\frac{3 s}{t_{1}+t_{2}+t_{3}}=\frac{3 s}{\frac{s}{v_{1}}+\frac{s}{v_{2}}+\frac{s}{v_{3}}}=\frac{3}{\frac{1}{v_{1}}+\frac{1}{v_{2}}+\frac{1}{v_{3}}} .
$$

Solution 2: Let the total length of the road be 3, then the lengths of the three different sections are all 1. Let $v_{1}=2, v_{2}=3, v_{3}=6$, then the time taken by the car to travel each section is $t_{1}=\frac{1}{2}, t_{2}=\frac{1}{3}, t_{3}=\frac{1}{6}$, and the average speed of the car over the entire road is
$$
\bar{v}=\frac{3}{t_{1}+t_{2}+t_{3}}=\frac{3}{\frac{1}{v_{1}}+\frac{1}{v_{2}}+\frac{1}{v_{3}}}=\frac{3}{\frac{1}{2}+\frac{1}{3}+\frac{1}{6}} .
$$

However, option (A) is $\frac{11}{3}$, (B) is $\frac{1}{3}$, and (C) is 1, all of which can be ruled out. Therefore, the answer is (D).