[填空题]People once thought of the concept of obtaining fresh water from icebergs. But at that time, it was only (26) a joke more appropriate to cartoons than real life. But now it is being considered quite (27) by many nations, especially since scientists have warned that the human race will outgrow its fresh water (28) faster than it runs out of food.
Huge masses of ice are a possible source of fresh water that has always been (29) until recently. Three-quarters of the Earth’s fresh water supply is still in the form of ice, a reservoir of fresh water so (30) that it could supply water to all the rivers of the world for 1000 years. Floating on the oceans every year are 7659 trillion tons of ice in ten thousand icebergs that break away from the ice caps of poles. Huge masses of ice that (31) the shallow continental shelf give birth to icebergs throughout the year. Icebergs are formed entirely on land, breaking off when

[填空题]People once thought of the concept of obtaining fresh water from icebergs. But at that time, it was only (26) a joke more appropriate to cartoons than real life. But now it is being considered quite (27) by many nations, especially since scientists have warned that the human race will outgrow its fresh water (28) faster than it runs out of food.
Huge masses of ice are a possible source of fresh water that has always been (29) until recently. Three-quarters of the Earth’s fresh water supply is still in the form of ice, a reservoir of fresh water so (30) that it could supply water to all the rivers of the world for 1000 years. Floating on the oceans every year are 7659 trillion tons of ice in ten thousand icebergs that break away from the ice caps of poles. Huge masses of ice that (31) the shallow continental shelf give birth to icebergs throughout the year. Icebergs are formed entirely on land, breaking off when

[单项选择]People once widely believed that intelligent life existed on Mars. The 19th-century discovery of what appeared to be geometric designs cut across the surface was taken as evidence. The lines were thought to have been a system of canals that had been built to irrigate the surface. It is now clear that the "canals"—perhaps the most spectacular geologic features of Mars—are natural valleys where ancient rivers once flowed. Another fragmented idea concerns the planet’’s seasonal changes in color. Once attributed to the rapid spread of some life-form, these shifts are now known to develop from the movement of fine dust in the atmosphere. By the close of the 20th century none of the many experiments conducted by spacecraft had ever found persuasive evidence of life. Nevertheless, speculation continued over the existence of some form of life, in either the present or past. In 1996 scientists discovered organic compounds and minerals in a meteorite (陨石), consisting of Martian rock, t
A. Because the surface of Mars seemed to be geometric.
B. Because the lines were drawn across the surface of the planet.
C. Because a system of canals was thought to be there.
D. Because it was the Martians that built the canals.

[单项选择]What did people once think about lightning
A. At open doorways.
B. In a closed car.
C. On the high ground.