Questions 51 to 54 based on the following passage:

　　Radioactive materials in wide use in the United States

　　could be turned into weapons terror that would probably

　　kill few people but would spread panic and produce severe

　　economic damage, scientists told the Senate Foreign

　　Relations Committee today.

　　The materials would not produce nuclear explosions.

　　Rather, they would be spread using conventional explosives

　　in so-called dirty bombs. The scientists who testified at

　　the committee’s hearing today said such weapons were a

　　potentially serious problem. The most dangerous materials,

　　they said, are bars of radioactive cobalt (钴) used to

　　irradiate（辐射） food, medical devices and other products

　　to sterilize (消毒) them. If turned into fine particles and

　　dispersed under the right weather conditions, the cobalt

　　could spread enough radiation to force the abandonment of

　　an area the size of Manhattan, the scientists said. But

　　federal and industry officials said such an action with

　　radioactive cobalt was unlikely because of the intense

　　radiation to which the person trying to use the weapon

　　would be exposed and because the material is transported in

　　secret and kept behind tons of lead and other shielding

　　while on the road or in use.

　　Other radioactive materials in tens of thousands of medical

　　and industrial devices could be used to make weapons that

　　would leave wide areas with radiation level above

　　Environmental Protection Agency limits, possibly leading to

　　temporary evacuation and extremely difficult cleanup.

　　Scientists at the hearing held to understand these threats

　　and to device ways of protecting Americans against them

　　also said that they remained concerned that fissionable(裂

　　变)material- the fuel for nuclear weapons-and the expertise

　　needed to use it could be spreading from the former Soviet

　　Union to terrorist networks, but presented no new evidence

　　of such leakage.

　　Dr. Richard Meserve, chairman of the Nuclear Regulatory

　　Commission, said that all such materials, as well as the

　　much more powerful cobalt rods used in sterilizing food,

　　are tightly controlled. He said that the weaker sources, if

　　dispersed, would cause almost immeasurably small increases

　　in cancer rates among people who were exposed.

　　Nevertheless, that sort of attack could cause long-term

　　problems because technique for dealing with radioactive

　　contamination rely largely on destruction and removal, Dr.

　　Henry Kelly, president of the Federation of American

　　Scientist, said. Because limits set by the Environmental

　　Protection Agency for radiation exposure are so low, areas

　　might have to be evacuated for the long term even when the

　　health risks are relatively slight, he said.

　　51. Which of the following can be the main idea of the

　　passage?

　　A. American should keep an eye on radioactive

　　materials that are likely be used for nuclear bombs.

　　B. America should keep an eye on cobalt bars that

　　are likely to be used for nuclear bombs.

　　C. America should keep an eye on radioactive

　　materials that are likely to be used for terrorism actions.

　　D. America should keep an eye on cobalt bars that

　　are likely to be used for terrorism actions.

　　52. Which of the words below can replace the word “fine”

　　(line 6, Para 2)?

　　A. Unnoticeable

　　B. Small

　　C. Good

　　D. Gaseous

　　53. Federal and industry officials are concerned with all

　　of the following BUT _____

　　A. Cobalt bars that can be used by terrorists to

　　launch terror actions against America.

　　B. Terrorist who may use cobalt bars to make nuclear

　　weapons for terror actions against America.

　　C. Radioactive materials in medical and industrial

　　devices that could be used to make weapons.

　　D. Terrorists who may get knowledge from other

　　countries to make nuclear weapons for terror actions.

　　54.as to the radiation exposure limits set by the

　　Environmental Protection Agency, Dr. Henry Kelly may think

　　that____

　　A. It’s necessary for the security of America

　　B. It’s too high so that it cannot function well.

　　C. It’s too low so that it can cause some

　　unnecessary costs and panic.

　　D. It’s a good limits system that function well for

　　anti-terrorism movement.