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An assembly plant accumulates its variable and fixed manufacturing overhead costs in a single cost pool, which is then applied to work in process using a single application base. The assembly plant management wants to estimate the magnitude of the total manufacturing overhead costs for different volume levels of the application activity base using a flexible budget formula. If there is an increase in the application activity base that is within the relevant range of activity for the assembly plant, which one of the following relationships regarding variable and fixed costs is true?