[单项选择]Despite the fact that comets are probably the most numerous astronomical bodies in the solar system aside from small meteor fragments and the asteroids, they are largely a mystery. Scientists do not know exactly what comets are or where they come from. Educated guesses are the best we have in hand.
Considering the role of comets in legend and the memory of man, it is remarkable that we still know so little, relatively, about them. The most famous comet of all, Halley’s Comet ( named for the man who predicted its return), was first sighted by the Chinese in 240 B．C．, and it has returned to terrify the people of the world on a regular basis every since then. The ancient considered it an object of ill omen. By mysterious coincidence, the arrival of Halley’s Comet coincided with such events as the battle of Hastings in 1066, the Jewish revolt of 66 A．D．, and the last battle of Attila the Hun against the Romans. Nor is it the only comet to fill man with awe, but merely the most famou
A. has no solid form
B. wanders like a frozen lump when it’s far out in space
C. requires the warmth of the sun to survive
D. is always followed by a long tail

[单项选择]

Passage Three
Despite the fact that today viruses(病毒) are known to cause cancer in animals and in certain plants, there exists a great reluctance to accept viruses as being of importance in human cancer. Basic biological phenomena generally do not differ strikingly as one goes from one species to another. It should be recognized that cancer :is a biological problem and not a problem that is unique for man. Cancer originates when a normal cell suddenly becomes a cancer cell which multiplies widely and without apparent restraint. Cancer may originate in many different kinds of cell, but the cancer cell usually continues to carry certain traits (特性) of the cell of origin. The transformation of a normal cell into a cancer cell may have more than one kind of cause, but there is good reason to consider the relationships that exist between viruses and cancer.
Since there is no evidence that human cancer, as generally experienced, is infections, many persons believe that
A. the fact that viruses have been known to mutate
B. the fact that a cancer-immune individual may lose his immunity
C. the fact that production of human cancer cells might be due to a genetic factor
D. the fact that man is host to many viruses

[单项选择]Passage 4 Despite the fact that today virus are known to cause cancer in animals and in plants, there exists a great reluctance to accept viruses as being of importance in human cancer. Basic biological phenomena generally do not differ strikingly as one goes from one species to another. It should be recognized that cancer is a biological problem and not a problem that is unique for man. Cancer originates when a normal cell suddenly becomes a cancer which multiplies widely and without apparent restraint. Cancer may originate in many different kinds of cell, but the cancer cell usually continues to carry certain traits of the cell of origin. The transformation of a normal cell into a cancer cell may have more than one kind of cause, but there is good reason to consider the relationships that exist between viruses and cancer. Since there is no evidence that human cancer, as generally experienced, is infectious, many persons believe that because viruses are infectious age
A. Viruses are known to cause cancer in animals and in plants.
B. Many people don’t believe that viruses cause human cancer.
C. Bioloigical phenomena don’t change much in animals and plants.
D. Cancer may originate in certain kinds of cells.