Which Statement Explains the Relationship Among Different Cost Functions

Which statement explains the relationship among different cost functions

A) The marginal cost of the fifth unit of output equals the total cost of five units minus the total cost of four units.
B) The average variable cost of seven units equals total fixed cost of seven units plus the total variable cost of seven units.
C) The marginal cost of the fifth unit of output equals the total fixed cost of five units minus the total variable cost of five units.
D) The total cost of seven units equals the total variable cost of seven units times seven.