Three costs incurred by Randall Rockers are summarized below at twodifferent 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 is variable?

A) A, B, and C

B) A and C

C) A only

D)C only

(I know the answer is B, but I can't figure out why. Can you please explain why only A and C are variable.)