阅读理解。阅读下列短文, 从给的四个选项 (A、B、C和D) 中, 选出最佳选项。
There are various reasons why cancers appear to be on the increase. For one thing, though their sufferers are to be found in all age groups, cancers are particularly likely to attack persons in their middle and advanced years. Naturally, since people live longer these years, there are more cancer sufferers than before. Again, with better methods of diagnosis (诊断), doctors can more easily recognize cancerous growths that would formerly have passed unnoticed or that would have been wrongly diagnosed. It is also believed that certain habits and conditions of modem living, including heavy smoking and the pollution of the air, may leave people living in more cancer-causing conditions than before.
We all look forward to the day when a simple medical test can find cancer while it is still small. Researchers around the world are working on such a test. Most of their work deals with the examination of the blood.
Researchers in Boston have found something in the blood of cancer patients that does not appear in healthy persons. The test showed which persons had cancer and which did not. It was correct more than 90 percent of the time.
