A doctor had two identical twins as patients. One suffered from lung cancer and the other did not. The twins had identical susceptibilityes to illness. One of the twins smoked cigarettes for decades whereas the other never smoked at all. The doctor concluded that the cigarette smoking probably caused the cancer. a) Difference b) Agreement c) Concomitant Variation d) Residues a) Difference b) Agreement c) Concomitant Variation ) Residues