a) Indicate each of the following costs as either fixed, variable, or semi-variable (i.e., mixed), and justify your choice. (3 marks) I. The cost is $500 per unit at a production level of 50 units, and $500 per unit at a production level of 100 units. II. The cost is $500 in total at a production level of 5 units, and $300 in total at a production level of 10 units. III. The cost is $500 in total at a production level of 50 units, and $1,000 in total at a production level of 100 units