All of the Following Statements Except One Are Correct

Multiple Choice

All of the following statements except one are correct.Which is the exception?

A) Firms always operate in the short run.
B) The law of diminishing returns applies in both the short run and the long run.
C) All inputs are variable in the long run.
D) As output increases,long run average cost could decline,remain constant or increase.
E) The long run can be described as a planning horizon.