72. A certain section of road consists of three equal-length segments: an uphill, a flat, and a downhill section. It is known that a car travels at average speeds of $v_{1}, v_{2}, v_{3}$ on these three segments, respectively. The average speed of the car over this section of 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. $\frac{3}{\frac{1}{v_{1}}+\frac{1}{v_{2}}+\frac{1}{v_{3}}}$
Answer: D