题目内容：
阅读材料，回答题。Alpha Particle
From decaying radon ( 氡) atoms can destroy the living cells they strike and increase the likelihood that those cells will later become cancerous.Researchers have now directly demonstrated that neighboring cells not suffering direct hits can be harmed, too. They've also taken a step toward showing how this type of radiation,called alpha particles, indirectly hurts those bystanders.
Radon derives from the decay of uranium ( 铀) and seeps naturally into the air from the ground.lt's the primary environmental source of alpha particles, which contribute to cancer risk by causing aberrations (失常)in DNA.Alpha particles from inhaled radon are second only to smoking as a cause of lung cancer.
Because a person's exposure to alpha particles typically is low, researchers have had to estimate public health threats from radon by guess from the effects of higher doses of alpha radiation.Such data comes primarily from studies of survivors of the atomic bombs that destroyed Hiroshima and Nagasaki in Japan. The customary extrapolation ( 推测 ) assumes that cancer risk is proportional to the dose of radiation even at low doses.
Radiation's effects in cell structure don't necessarily reflect what happens in "a whole organism, with its full range of defense repair mechanisms, " says Duport.Processes such as DNA repair and cell death triggered by radiation damage could cancel the effect on by stander cells observed in the lab, he suggests.
Furthermore, while a bystander effect can contribute to cancer, other cell-to-cell interactions in living tissue "may relieve increased risk." says Barry Michael, a radiation biophysicist at the Gray Cancer Institute in Northwood, England.One of these interactions halts cell division and hence cancer. "The jury is still out on whether cell-to-cell effects lead to a greater or lower risk, " Michael says.
The passage's main topic is__________. A．The experiment done by researchers
B．Uranium is the key killer of neighboring cells
C．Cell-to-cell interaction can make up for the hurt cells
D．Bystander cells can be indirectly damaged by alpha particles
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