World's Best Chair Co

World's Best Chair Co.has produced 30 chairs at a total cost of $300.The next chair produced will add $10 to the company's total cost.Which of the following is definitely true if the 31st chair is produced?

A) Short-run average total cost will decrease.
B) Marginal cost will decrease.
C) Average fixed cost will decrease.
D) Both A and C are true.