閱讀下面短文，根據(jù)所讀內(nèi)容在文后第76至第85小題的空格里填上適當(dāng)?shù)膯卧~或短語(yǔ)。注意：每空不超過(guò)3 個(gè)單詞。

It is difficult to imagine what life would be like without memory. The meanings of thousands of everyday perceptions, the bases for the decisions we make, and the roots of our habits and skills are to be found in our past experiences, which are brought into the present by memory.

Memory can be defined as the capacity to keep information available for later use. It includes not only remembering things like arithmetic or historical facts, but also involves any change in the way an animal typically behaves. Memory is involved when a rat gives up eating grain because he has sniffed (嗅，聞) something suspicious in the grain pile. Memory is also involved when a six-year-old child learns to swing a baseball bat.

     Memory exists not only in humans and animals but also in some physical objects and machines.

Computers, for example, contain devices for storing data for later use. It is interesting to compare the memory-storage capacity of a computer with that of a human being. The instant-access memory of a large computer may hold up to 100 000 words ready for instant use. An average U. S. teenager probably recognizes the meaning of about 100,000 words of English. However, this is but a fraction of the total amount of information which the teenager has stored. Consider, for example, the number of faces and places that the teenager can recognize on sight.

The use of words is the basis of the advanced problem-solving intelligence of human beings. A large part of a person's memory is in temps of words and combinations of words.

    Scientists believe that conditions on Mars around 3.8 billion years ago were very similar to those of the early earth, when primitive organisms were spreading through our oceans. At that time, Mars would have been much warmer and wetter than it is today, with an atmosphere composed mainly of carbon dioxide, just like that of the earth at the time. Under these conditions, it is highly probable that life may have arisen on Mars as well.

     But, even if life did not arise naturally on Mars, it does not mean that it could not have existed there.  According to Professor Paul Davies of the University of Adelaide, Australia, life forms could have been transferred between the earth and Mars in wreckage (broke pieces) created by the impact of comet (彗星) and small planets on the surface of the two planets.

     Even today, about 500 tons of material from Mars lands on earth every year. It is mainly in the form of the dust but occasionally a larger chunk (厚片, 大塊) strikes the earth. In 1911, a piece of Martian rock crashed in Egypt, killing a dog. According to professor Davies, it is in these chunks of rock, which were much larger and more frequent in the past, that life forms could have been transported from planet to planet.

     "But how could these life forms have survived their journey through space?" Says Paul Davies, "The difficulty in believing this theory is that a bacterium on its own in space has to struggle itself not only against cold but also against deadly cosmic (宇宙的) radiation. But wrapped in a rock the situation is different. A rock ten meters across would shield life inside it from a lot of radiation and the temperature might only be minus 10 or 20 degrees, the sort of thing we have on earth."

73. Where do some scientists suppose life probably come from? （回答不超過(guò)6個(gè)單詞）

74. What does Professor Paul Davies believe? （回答不超過(guò)6個(gè)單詞）

75. Why could life survive when transferred from one planet to another? （回答不超過(guò)12個(gè)單詞）