Which of the Following About Costs Is Always True

Which of the following about costs is always true?

A) When marginal costs are less than average total costs, average total costs will be decreasing.
B) When average fixed costs are falling, marginal costs must be less than average fixed costs.
C) When average fixed costs are rising, marginal costs must be greater than average total costs.
D) When marginal costs are greater than average total costs, average total costs will be decreasing.