A curvilinear relationship between variable costs and changes in activity levels suggests what? a. A strictly linear relationship between fixed costs and activity levels is implausible. Ob. All of the above. Since the relationship between activity levels and variable costs is linear within the relevant range and less linear at lower and higher levels outside the relevant range, the straight-line (linear) relationship takes on a curvature in the real world. d. A strictly curvilinear relationship between changes in activity levels and variable costs is possible only within the relevant range