When the activity level of a firm changes, _____. a. the total fixed costs of a firm will be constant for all possible mixes and are not relevant to the decision b. the total fixed costs of a firm will vary according to the possible mixes and are relevant to the decision c. the total variable costs of a firm will vary according to the possible mixes and are not relevant to the decision d. the total variable costs of a firm will be constant for all the possible mixes and are relevant to the decision