Which of the following statements regarding a company's sales mix is not true?

A shift in the sales mix from products with high contribution margin ratios to products with low contribution margin ratios will result in a decrease in operating income.

The cost$-$volume$-$profit analysis model assumes that a company's sales mix will remain constant as sales volume increases or decreases.

For purposes of performing cost$-$volume$-$profit analysis, the overall contribution margin is defined as the simple average of each product line's contribution margin ratio.

A shift in the sales mix from products with high contribution margin ratios to products with low contribution margin ratios will increase the break$-$even point for the company.