Use the graph to answer the question. The graph shows the relationship between the time and the speed of a train that has already left the station.
Train Speed
Time (minutes)
During which time period was the train traveling at a constant speed?
A
between 0 and 20 minutes
B. between 20 and 40 minutes
C
between 40 and 60 minutes
D. between 60 and 80 minutes