A sample 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. BOTH of the test tubes were kept for 8 hours at 40 degrees C. These conditions are called

A; Constants
B: Experimental groups
C:Factors
D:Control groups