A person walks from his house to this office at a speed of X1 km/h and return by the same route at the speed of X2 km/h. His average speed is

• A. \(\frac{X_1 + X_2}{2}\)
• B. \(\frac{X_1 + X_2}{2}2\)
• C. \(\frac{3}{4} \ (\frac{X_1X_2}{X_1 + X_2})\)
• D. \(\frac{{2{{\rm{X}}_1}{{\rm{X}}_2}}}{{{{\rm{X}}_1} + {\rm{\;}}{{\rm{X}}_2}}}\)