Spontaneous generation is the hypothesis that nonliving material can spontaneously change into a living organism. Hundreds of years ago, many scientists accepted this hypothesis. The following is one experiment that was performed to test the hypothesis of spontaneous generation: Meat was placed in three identical glass jars. One jar was left open, one jar was tightly sealed, and one jar was covered with a fine mesh that allowed air into the jar but kept flies out. Maggots (fly larvae) only appeared in the open jar. Was the control used in this experiment adequate?  A.  No, this experiment included two variables instead of one.  B.  Yes, the open jar acted as a control.  C.  No, this experiment did not include a control group.  D.  Yes, both the sealed jar and the jar with mesh were controls for the open jar