Choice "B" is correct. Break-even analysis assumes that all variable costs and revenues are constant on a per unit basis and are linear over a relevant range. Fixed costs in total are constant.
Choice "d" is incorrect. Break-even analysis assumes that all variable costs and revenues are constant on a per unit basis and linear over a relevant range.
Choice "a" is incorrect. Total costs do change over a relevant range. Break-even analysis assumes that all variable costs and revenues are constant per unit and linear within a relevant range.
Choice "c" is incorrect. Total fixed costs are assumed to be constant (representing a linear relationship) over a relevant range.