Student Bio Microscope

another BIO question...PLEASE?!! Quick ten!! THANK YOU!!?

a student was studying colony of yeast cells under microscope...after third hour, number of yeast cells decreased (3rd hour - 45 cells, 4th hour - 30 cells, 5th hour - 8 cells)
why does this happen?

thank youuuuuuu!!

Either they had run out of food in the medium, or the waste that they produce like alcohol (if they are doing anaerobic respiration) is toxic to themselves and at a high population density enough could be produced that at sufficient concentration will kill them.