Which of the following best describes directly proportional relationships?

A directly proportional relationship is defined by the behavior where two quantities increase or decrease together at a constant ratio. When one value becomes larger, the other value also increases in a linear manner, maintaining the proportion between them. Conversely, if one value decreases, the other also decreases, again maintaining that set ratio. This consistent relationship characterizes direct proportionality.

In contrast, the first choice describes an inverse relationship where the values move in opposite directions. The second option similarly indicates that the two values change in opposite directions, which does not align with direct proportionality. The last choice refers to a constant relationship where the values do not change at all, which also does not represent the principle of direct proportionality where the values are variable but consistently related to each other.