KPQ Enterprises is using the high$-$low method to analyse its overhead costs. Last quarter, the company operated at $2000$ units with total overhead costs of R$120000$ and at $4000$ units with overhead costs of R$180000.$

What critical assumption must the management accountant consider to ensure this method provides accurate insights, and why?

A$.$ All production levels should yield the same overhead costs, ensunng that costs remain unaffected by production volume.

B$.$ Variable costs per unit should vary based on economies of scale as this provides a more realistic picture of actual costs.

C$.$ The relationship between units produced and total overhead costs must remain linear, as non$-$inearity would distort the fixed and variable cost estimates.

D$.$ Fixed costs should increase proportionately with production levels, ensuring overhead is allocated correctly.