A company charges $6$ per unit for its product and has these total costs. Output, in units: $0, 10, 20, 30$. Total costs ($\$$): $40, 100, 120, 150$. Which of the following statements is correct?
- AAverage cost is lowest when 10 units are produced.
- BThe firm does not make any profit when 20 units are sold.
- CThe firm has no fixed costs.
- DTotal variable costs fall continuously over these outputs.