One difference between the long run and the short run in a perfectly competitive industry is that

A、economicprofitsinthelongrunarealwaysgreaterthantheyareintheshortrun.;
B、economicprofitsintheshortrunarealwaysgreaterthantheyareinthelongrun.;
C、firmsnecessarilyoperateattheoutputlevelthatminimizesaveragetotalcostonlyinthelongrun.;
D、marginalrevenueequalspriceonlyinthelongrun.