What defines a direct proportion between two quantities?

A direct proportion between two quantities is characterized by a consistent ratio between them, meaning that as one quantity changes, the other quantity changes at a predictable rate. This relationship can be expressed mathematically as ( y = kx ), where ( k ) is a constant of proportionality. When both quantities change in a set ratio, if one quantity doubles, the other quantity also doubles, and this relationship remains true for any proportional change. This concept is fundamental in understanding relationships in mathematics, science, and economics, where proportional relationships are often observed.

The other options suggest different types of relationships that do not represent direct proportion. For instance, one quantity decreasing while the other increases indicates an inverse relationship, while constant quantities would imply no change in relation. Lastly, a non-linear relationship does not meet the criteria of direct proportion, as it does not maintain a constant ratio across changes in the two quantities.