The following data show the relationship between total costs and output in the short run.

Output Total Cost
0 $5
1 8
2 12
3 15
4 20
5 27

The firm's marginal costs are equal to average total cost somewhere between units

A. 1 and 2.
B. 2 and 3.
C. 3 and 4.
D. 4 and 5.