Although Parkinson's disease may be caused by a number of factors, doctors speculate that Ali contracted the disease as a result of receiving too many blows to the head. So, in Ali's case, to which of the following factors can his disease best be attributed?

Asample of digestive juice was removed from the stomach of a pig. the juice was placed in a test tube along with some grains of wheat. a second test tube was set up containing an equal number of wheat grains, with distilled water rather than digestive juice. the test tubes were kept for 8 hours at 40 degrees c. after 8 hours, the grains inside the digestive juice had broken into tiny particles. the grains of wheat, in the water, were wet but otherwise unchanged. what is the control group?