Certain diseases, including bovine spongiform encephalopathy ("mad cow disease") and Creutzfeldt-Jakob disease, are caused by an infectious agent whose nature was at first a mystery. Similar infectious agents are also known to cause certain telltale traits in yeast. Under certain conditions, the infected yeast cells turn red. (Normal yeast cells arewhite.)

Suppose you are studying a strain of redcolored yeast cells that you believe contains an infectious agent such as bovine spongiform encephalopathy. You first prepare a extract of the red cells that contains the cells' DNA, RNA, and protein. Then you mix the extract with normal white cells to find out whether the red cell extract can convert normal white cells to red. In this experiment, the experimental treatment is mixing the red cell extract with normal whitecells.

The proper negative control experiment would be to mix normal white cellswith:

an extract from normal whitecells.

sterile saltsolution.

living whole redcells.

nutrient medium.