Which one of the following is a correct assumption that underlies CVP analysis?

Explain your choice.

$1-$ For costs classified as variable, the costs per unit of output must change constantl

$2-$ For costs classified as fixed, the costs per unit of output must remain constant.

$3-$ All costs behave predictably within the relevant range.

$4-$ The selling price per unit must change in proportion to the number of units sold.

The following information is available for a company's maintenance cost over the

last seven months. Using the high$-$low method, estimate both the fixed cost per

month and variable cost per unit for maintenance cost.