In the past, overhead costs were allocated based on either direct labor hours or machine hours. Why is it becoming less popular to use only one cost allocation base?

Select one:

a$.$ Since direct labor is cheap, it properly estimates manufacturing overhead

b$.$ It is too accurate

c$.$ Since more and more companies are producing a variety of products in the same plant, it has become increasingly difficult to rely on only one cost allocation base

d$.$ Machines are no longer used