If the slope of a displacement-time graph is constant, it means that the object's velocity is constant. Therefore, this option is not correct.
When the slope of a displacement-time graph is constant, it indicates that the velocity of the object is constant. So, this is the correct option.
If the slope of the graph is constant, it implies a constant velocity. So, this option is not correct.
A constant slope on a displacement-time graph does not indicate infinite velocity. So, this option is not correct.
The correct answer is B. When the slope of a displacement-time graph is constant, the velocity of the object is also constant.