All of These Are an Assumption of CVP Analysis $\bold{except:}$ A)fixed Costs Remain Constant Over the Relevant Range

All of these are an assumption of CVP analysis, $\bold{except:}$

A) fixed costs remain constant over the relevant range.
B) variable costs per unit remain constant over the relevant range.
C) the sales mix for a multi-product producer is constant.
D) as sales increase,the selling price must decrease.