The Data in the Table Is for a Firm in Which

The data in the table is for a firm in which labour is the only variable cost and each unit of labour costs the same regardless of how many units are bought.

-Refer to the information above to answer this question.As one reads down the MC column,what is the pattern in the numbers?

A) They fall until the fourth unit of labour used and then rise.
B) They fall until the fifth unit of labour used and then rise.
C) They rise until the fourth unit of labour used and then fall.
D) They rise until the fifth unit of labour used and then fall.
E) They continually rise.