One reason for the appearance of diminishing returns is that:
A. whenever inputs are added to a production process, output falls because of increased inefficiency
B. the fixed inputs cannot produce any more as more units of a variable input are added
C. each additional unit of an input has proportionately less of the other units with which to work
D. the marginal product continually rises whenever output is increased
E. specialization becomes impossible as more units of a variable input are added to a fixed input