Notice that the graph is about time-position, which means every horizontal segment means the position didn't change, in other words, the velocity was null.
The other segments indicate a constant velocity. During periods A and C, the constant velocity was positive, during E, the constant velocity was negative.
The reason why those segments indicate a constant velocity is because they are described with a line, which has a constant rate of change.
Therefore, the answers are A, C, and E.