Answer:
The direction in which this object travels might be changing.
Explanation:
Velocity is a vector quantity. In addition to the rate at which this object moves, velocity also measures the direction of the motion.
In contrast, speed is a scalar quantity. The speed of an object also measure the rate at which this object moves. However, unlike velocity, speed does not account for the direction of the motion.
Hence, even if speed is constant, velocity of this object might not be constant as long as the direction of the motion is changing.
For example, consider an object moving around a circle at a constant rate. The speed of this object will be constant since the rate of the motion is constant. However, the velocity of this object will not be constant since the object needs to keep changing its direction to stay on the track.