【文章內(nèi)容簡介】 rest of my mountain, I watched an ant city prepare to send forth its young queens. At the precise moment that they took wing, a colony of the same species that my wife was watching near the bottom of the mountain, also sent its queen on a wedding flight. There was, of course, no way could the two colonies have checked take off time with each other.Entomologist Albro T. Gaul once jotted down in his notebook that a particular the same time! This splitsecond timing is not always the rule. However, most flights take place within a definite period of time.Birds also have builtin timepieces which send them off on fall and spring migrations. What the birds really have is a clock like mechanism which allows them to time hours of darkness or light in each day.But what sends birds northward again in the spring? New research by Dr. Albert Wdifson of Northwestern University seems to indicate that the timing of return flight is extraordinarily plex. In the fall of the year the short days and long nights cause the clocks in migratory birds to undergo a kind of winding in preparation for their spring return and breeding. Then during the late fall and winter as the clock ticks, certain physiological changes occur in the bird. The length of each day during the winter determines how fast the clock will run, and hence when the alarm will ring for the spring migration. The clock continues to run through breeding time, then stops—to be rewound again the next fall.45. The alarm clock that determines the activity of certain living things is governed by ____.A. hours of daylight B. the time of day in their native environmentsC. the moon D. something we don39。t understand pletely46. The fiddler crab seems to darken his color according to ____.A. time of sunrise B. its backgroundC. amount of daylight D. time of tides47. The reported activity of the ant colony occurred in relation to ____.A. the position of the sun B. the day of the yearC. the temperature D. the geographical location48. What controls the migration of birds seems to be ____.A. dark, cloudy days and bright, sunny daysB. direction of migrationC. time between sunrise and sunsetD. breeding habitsQuestions 49 to 52 are based on the following passage:Medicine es in many forms. In its liquid form, medicine affects the body very quickly. But the effects of liquid medicine aren39。t usually long lasting. That is why pills and capsules are also used.The pills and capsules being sold today aren39。t perfect, either. Pills dissolve in the stomach. The medicine in the pills is released when the pills dissolve. But often, the pills dissolve too quickly.Scientists have been trying to develop a pill that can release medicine slowly over a long period of time. They have applied their knowledge of plants to produce the “osmotic (滲透的) pump pill”.The cell walls of plants are made of cellulose (纖維素). Cellulose is a very porous substance. There are millions of tiny holes, or pores, in the cellulose walls of plants. These holes are big enough to allow water through the cell walls. As water enters a cell, pressure builds up in the cell. The pressure pumps other substances out of the cell. These substances leave the cell through the cellulose wall. This slow, steady process is called osmosis.The osmotic pump pill is coated with synthetic cellulose. Liquid medicine is contained in the pill. The holes in the cellulose coating of the pill are big enough to allow water in the pill. As water from the body enters the pill, pressure builds up and the medicine is then slowly pumped out of the pill.49. The passage implies that the osmotic pump pill is better than other pills and capsules because____.A. it releases medicine slowly over a long period of timeB. the coating doesn39。t dissolve in the stomachC. the medicine in the pill can affect the body quicklyD. it helps to build pressure in the body50. The way that the osmotic pump pill works is based on a process called ____.A. cellulose B. osmosis C. pressure D. synthesis51. The passage implies that medicine in an osmotic pump pill will leave the pill when ____. A. the pill is swallowed B. the cellulose coating is dissolved C. enough pressure builds up in the pill D. the medicine is dissolved with water from the body52. The passage implies that cellulose is a very porous substance because it contains ____.A. millions of tiny holes B. a substance that dissolves it C. a substance that creates pressure D. liquid medicineQuestions 53 to 56 are based on the following passage:Prices determine how resources are to be used. They are also the means by which products and services that are in limited supply are rationed among buyers. The price system of the United States is a very plex network posed of the prices of all the products bought and sold in the economy as well as those of a myriad of services, including labor, professional transportation, and publicutility services. The interrelationships of all these prices make up the system of prices. The price of any particular product or service is linked to a broad, plicated system of prices in which everything seems to depend more or less upon everything else.If one were to ask a group of randomly selected individuals to define price, many would reply that price is an amount of money paid by the buyer to the seller of a product or service or, in other words, that price is the money value of a product or service as agreed upon in a market transaction. This definition is, of course, valid as far as it goes. For a plete understanding of a price in any particular transaction, much more than the amount of money involved must be known. Both the buyer and the seller should be familiar with not only the money amount, but also with the amount and quality of the product or service to be exchanged, the time and place at which the exchange will take place and payment will be made, the form of money to be used, the credit terms and discounts that supply to the transaction, guarantees on the product or service, delivery terms return pr