[不定项选择题]共用题干 Rainmaking
1 The idea of rainmaking is almost as old as man,but it was not until 1946 that man succeeded in making rain.In ancient times,rainmakers had claimed to bring rain by many methods:dancing.singing-Kiiiing animals t including humans).
2 For a long time,men have understood where rain comes from.Water from the surface of oceans and lakes becomes part of the air,where it forms clouds from which rain falls.But exactly what starts the formation of raindrops was not known until quite recently.A man named John Aitken proved that drops of water gather around tiny bits of dust or other matter.The centers of the drops are so small that the human eye cannot see them.Without such centers,it seems raindrops do not form.
3 During World War Ⅱ,Dr.Irving Langmir,and his assistant Schaefer,were hired by the General Electric Company to study how and why ice forms on the wings of airplanes.They went to a mountain in New Hampshire,where snowstorms are common and cold winds blow.They were surprised to find that often the temperature of the clouds surrounding them was far below the freezing point,and yet ice did not form in the clouds.
4 After the War,Schaefer experimented with a machine that created cold,moist air similar to the air found in clouds.To imitate the moist air of a cloud,Schaefer would breathe into the machine.Then he would drop into the freezer a bit of powder,sugar,or some other substance.For weeks and months he tried everything he could imagine.Nothing happened.No crystals of ice were formed.None of the substances would serve as the center of a snow crystal or raindrops.
5 One July morning,Schaefer was dropping in bits of various substances and watching the unsuccessful results.Finally,a friend suggested that they go to eat lunch and Schaefer went with him.As usual,he left the cover of the freezer up,since cold air sinks and would not escape from the box.
6 Returning from lunch,Schaefer found that the temperature of the freezer had risen to a point higher than that required for ice crystals to remain solid.There were two choices now.He could close the cover and wait for the freezer itself to lower the air temperature,or he could make the process occur faster by adding dry ice-a gas in solid form that is very,very cold.He chose the latter plan.As he dropped the steaming white dry ice into the freezer,he happened to breathe out a large amount of air.And there,before his eyes,it happened!He had made ice crystals,not by adding centers to the moisture,but by cooling the breath so much that the liquid had to form crystals!Then he began to blow his breath into the freezer and drop large pieces of dry ice through it to create crystals which became a tiny snowstorm falling slowly to the floor of his laboratory.
7 After planning carefully,Schaefer made an experiment by dropping dry ice from his plane to the clouds below him.As was expected,snow formed and fell from the bottom of the cloud.Schaefer succeeded.He made history. Schaefer successfully made a field experiment on rainmaking after planning______.
A.imaginatively
B.recently
C.carefully
D.unscientifically
E.accidentally
F.satisfactorily