Which of the following is not true?

Select one:

a. It is generally assumed that variable costs remain approximately the same per unit sold as the volume of sales changes from the busy to the quiet season in a year.

b. It is generally assumed that total variable costs increase when sales increase.

c. It is generally assumed that total monthly fixed cost remains the same as the volume of sales changes from the busy to the quiet season in a year.

d. It is generally assumed that fixed cost per unit remains the same when sales decrease.