Which of the following is not correct regarding variable costs and fixed costs.
a. Variable costs change in proportion to changes in the related level of activity while fixed costs remain unchanged irrespective of the level of activity within the relevant range.
b. Variable costs include costs of materials and factory wages while fixed costs include salaries paid to office staff.
c. Variable costs remain variable throughout the production period and fixed costs remain fixed at all times.
d. Variable costs can be reduced by reducing the level of activity while fixed cost cannot be reduced even if the level of activity is reduced.