1. Cost-volume-profit analysis is based on certain general assumptions. Which of the following is   not   one of these assumptions?
(A) Product prices will remain constant as volume varies within the relevant range.
(B) Costs can be categorized as fixed, variable, or semivariable.
(C) The efficiency and productivity of the production process and workers will change to reflect manufacturing advances.
(D) Total fixed costs remain constant as activity changes.
(E) Unit variable cost remains constant as activity changes.