According to the question,

A=60, B=30, C=10

we know the formula,

Average speed during the whole journey =$\frac{3\mathrm{ABC}}{\text{AB+BC+CD}}$

Average speed the whole journey

$$\begin{gathered} =\frac{3\times 60\times 30\times 10}{60\times 30+60\times 10+30\times 10} \\ =\frac{54000}{2700}= 20 \mathrm{km}/\mathrm{hr} \end{gathered}$$

Hence, Option C is correct.