QUESTION 15 Three costs incurred by Randall Rockers are summarized below at two different activity levels, 600 and 700 units: 600 Units 700 Units Cost A $2,700 $3,150 Cost B 2,940 3,360 Cost C 4,800 5,600 Which of these costs is variable? (hint...use the definition of variable cost per unit) Oa. Conly b. A only c. A, B, and C d. A and C