$10.$ Which of the following statements regarding costs and decision making is correct?

a$.$ Fixed costs must only be considered on a per$-$unit basis.

b$.$ Fixed costs will actually behave as variable costs when calculated on a per$-$unit basis for decision making.

c$.$ Per$-$unit fixed cost amounts are valid only for make$-$or$-$buy decisions.

d$.$ Per$-$unit fixed costs can be misleading because such amounts appear to behave as variable costs when, in actuality, the amounts are related to fixed expenditures.