问题
单项选择题 A型题
射野中心轴一般指的是（）
A.源中心与照射野几何重心两点连线
B.源中心与照射野中心两点连线
C.源中心与照射野剂量计算点两点连线
D.源中心与照射野剂量归一化点两点连线
E.源中心与准直器中心两点连线
答案
参考答案：B
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射野中心轴一般指的是（）
A.源中心与照射野几何重心两点连线
B.源中心与照射野中心两点连线
C.源中心与照射野剂量计算点两点连线
D.源中心与照射野剂量归一化点两点连线
E.源中心与准直器中心两点连线
参考答案：B
| It sounds like a science fiction, but researchers say it’s a scientific fact: Microscopic organisms dubbed "killer algae" are paralyzing fish with unknown toxins and then eating away at their flesh. They might be making people sick, too. The name given to this single-cell organism, or dinoflagellate, is Pfiesteria piscimorte—literally, "fish killer." It was discovered at North Carolina State University in 1988, but at the same time, few scientists believed in its existence, much less in its highly unusual predatory nature. But ongoing research has led to international acknowledgment of the phenomenon and, recently, research funding. Seemingly prompted by an unknown substance secreted by fish, the aggressive creature swims into action. It sends neurotoxins into the water and air, paralyzing a fish’s nervous system, and causing it to gasp for air at the surface. Eventually the fish suffocates. The killer dinoflagellate then attaches itself to the fish and begins sucking away at its flesh. This macabre scenario may help solve several sea mysteries, such as why fishermen report seeing "fish walks" (fish trying to leave the water) and pools of dead fish with holes eaten through them. Whether humans are affected by the dinoflagellate’s toxin remains to be seen. Researchers think it’s unlikely eating fresh fish are in danger, but anyone with frequent exposure to the creature could be in peril. Several researchers working with the algae have reported bouts of memory loss and disorientation. To understand the killer algae further, scientists must fully characterize their toxin and determine what stimulates them to attack. They also need to know if the algae are hurting fish populations, and whether pollutants make them more likely to attack. |
Which of the following statements is tree ______ .
A．Scientists have solved the mystery about the fish killer.
B．More studies need to be made about the fish killer.
C．It is proved that the algae are reducing fish populations.
D．Humans have already been affected by the toxin.