Buerger病坏死期的临床表现 A．间歇性跛行 B．“5P”征 C．(指)趾端发黑

Far from the land of Antarctica, a huge shelf of ice meets the ocean. At the underside of the shelf there lives a small fish, the Antarctic cod.

For forty years scientists have been curious about that fish. How does it live where most fish would freeze to death? It must have some secret. The Antarctic is not a comfortable place to work and research has been slow. Now it seems we have an answer.

Research was begun by cutting holes in the ice and catching the fish. Scientists studied the fish’s blood and measured its freezing point.

The fish were taken from seawater that had a temperature of -1.88℃ and many tiny pieces of ice floating in it. The blood of the fish did not begin to freeze until its temperature was lowered to -2.05℃. That small difference is enough for the fish to live at the freezing temperature of the ice-salt mixture.

The scientists’ next research job was clear: Find out what in the fish’s blood kept it from freezing. Their search led to some really strange thing made up of a protein(蛋白质) never before seen in the blood of a fish. When it was removed, the blood froze at seawater temperature. When it was put back, the blood again had its antifreeze quality and a lowered freezing point.

Study showed that it is an unusual kind of protein. It has many small sugar molecules(分子) held in special positions within each big protein molecules. Because of its sugar content, it is called a glycoprotein. So it has come to be called the antifreeze fish glycoprotein, or AFGP.

49. What is the text mainly about?

A. The terrible conditions in the Antarctic.          B. A special fish living in freezing waters.

C. The ice shelf around Antarctic.                  D. Protection of the Antarctic cod.

50. Why can the Antarctic cod live at the freezing temperature?

A. The seawater has a temperature of -1.88℃.    

B. It loves to live in the ice-salt mixture.

C. A special protein keeps it from freezing.        

D. Its blood has a temperature lower than -2.05℃.

51. What does the underlined word “it” in Paragraph 5 refer to?

A. A type of ice-salt mixture.                      B. A newly found protein.

C. Fish blood.                                           D. Sugar molecule.

52. What does “glycol” in the underlined word “glycoprotein” in the last paragraph mean?

A. sugar                      B. ice                   C. blood                      D. molecule

进给驱动用的伺服电动机主要有步进电动机、直流和交流调速电动机，交流调速电动机是机床进给驱动发展的一个方向。

伽马刀的主要部件有（）

A.治疗机，包括上半球形防护罩和中央部的机体

B.治疗床和移床装置

C.4种不同规格的准直器头盔

D.控制装置

E.宽度为1cm的40对的多叶准直器

简述果岭草坪的修剪要求