The diagram shows the speed-time graph for a bicycle journey. During which part is the bicycle moving at a constant speed?
- Abetween $0$ and $20\,\text{s}$
- Bbetween $20\,\text{s}$ and $40\,\text{s}$
- Cbetween $40\,\text{s}$ and $50\,\text{s}$
- Dbetween $50\,\text{s}$ and $70\,\text{s}$