单项选择题

A daily dose of aspirin can prevent cancer in people with a genetic disorder that increases their risk of developing the disease, scientists said on Monday.
The finding could also have important implications for the wider population, although more research is needed and unraveling the connection will take some time since the benefits of aspirin were only seen after several years.
John Burn of the Institute of Human Genetics at Newcastle University in Britain said his study might also have uncovered a simple way of controlling stems cells that make tumors grow.
"We believe that aspirin may have an effect on the survival of aberrant (异常的) stem cells in the colon," Burn said, presenting his findings at the ECCO-ESMO European cancer congress in Berlin.
Burn and colleagues tested 1,071 people with Lynch syndrome (林奇综合症) — an inherited condition that predisposes a person to a range of cancers, particularly of the colon — by giving some of them aspirin and some a placebo (实验对照药).
Follow-up tests after 10 years showed that although there was no difference in cancer rates after 29 months, a significant difference was detected after four years, with fewer people in the aspirin group developing colon cancer, Burn said.
"To date, there have been only six colon cancers in the aspirin group as opposed to 16 who took placebo," he added.
Burn said that although people in the trial stopped taking aspirin, its effect clearly continued.
Colorectal is the second biggest cause of cancer death in the United States and Europe, where a total of 560,000 people develop the disease each year, and 250,000 die from it.
Aspirin, originally developed by Bayer, is a cheap over-the-counter drug which in low daily doses has been found to stave off the risk of heart attacks and strokes, as well as chase away occasional aches and pains.
Other scientists have previously found it can reduce the risk of developing colon cancer and suggested it does so by blocking the COX-2.
But Burn said he thought this explanation was unlikely, and thinks that aspirin hits faulty stem cells before they mutate into pre-cancerous cells.
"If aspirin reduced the chances of such cells surviving, this would explain our results," he said.
Despite its benefits, aspirin is also well known for causing stomach upsets. In the study, 11 patients on aspirin had stomach bleeds or ulcers (溃疡) compared with nine on placebo.
The team plans a further study using a larger group of patients taking differing aspirin doses.
According to the passage, the death rate of colorectal in the United States and Europe is about ______.