Reading the following passage and choose the correct answer(s). Six children were very sick in the hospital and Dr. Bessor, an expert on strange illnesses, was called to find the cause of the illness. Through his microscope, he managed to spot a strain of the bacteria E. Coli, which he later found very dangerous to humans. Dr. Bessor knew E. Coli could move from animals to humans. He found other E. Coli cases in teh area where the children lived, but it wasn't enough. Besser then made a list of what the sick children had eaten. They had all eaten cheese, apple juice, ad fish. And made a list of what healthy children in teh area had eaten, and compared his lists. They had eaten the cheese and fish, but not the apple juice. Finally, Dr. Besser went to where the apple juice was made, and he saw animals around apple trees, working using dirty appleas that had fallen on the ground and making juice without washing apples. More importantly, the juice was not heated. Based on these findings, Dr. Besser concluded that it was teh happle juice that made the children sick. What method did Dr. Besser use to identify the cause?