In Economic Theory,if an Additional Worker Adds Less to the Total

Multiple Choice

In economic theory,if an additional worker adds less to the total output than previous workers hired,it is because

A) there may be less that this person can do,given the fixed capacity of the firm.
B) he/she is less skilled than the previously hired workers.
C) everyone is getting in each other's way.
D) the firm is experiencing diminishing returns to scale.