A、A controlled study. B、A trial study. C、An observational study. D、A successful study. C文章最后提到Chan博士将使用阿斯匹林对结肠癌病人进行治疗的研究成果，发布在美国

People since ancient times have used aspirin-like medicines to fight pain and reduce high body temperature. (26)Modern research has found other uses of aspirin. The drug acts as a blood thinner. It can help blood flow past a blockage in an artery. Blockages can cause heart attacks or strokes. As a result, patients at risk of blockages might be advised by their doctors to take a low-strength aspirin every day.
     And research continues.  (27)  A new study has shown that aspirin can improve survival in colon cancer patients. It involved about one thousand three hundred patients with colorectal cancer.  The cancer had not spread to other parts of the body yet. The study compared patients who took three hundred twenty-five milligrams of aspirin at least two times a week with those who did not use aspirin. The study found that the aspirin users had an almost thirty percent lower risk of dying from their cancer. That was during an average of eleven years after the cancer was discovered.
     Andrew Chan of Harvard Medical School and Massachusetts General Hospital led the study. Doctor Chan says the effects appeared especially strong among patients with tumors expressing a chemical called COX-2. Two-thirds of colorectal cancers produce that chemical. Doctor Chan thinks the aspirin works by blocking it. (28)The study appeared last week in the Journal of the American Medical Association. It was an observational study. In a controlled study, some patients would have taken aspirin. Others would have received a placebo—sugar pills—for comparison.