(Viên chức Mầm non) 650 câu trắc nghiệm Đọc hiểu tiếng Anh có đáp án - Phần 9

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The mineral particles found in soil range in size from microscopic clay particles to large boulders. The most abundant particles - sand, silt, and clay -are the focus of examination in studies of soil texture. Texture is the term used to describe the line composite sizes of particles in a soil sample, typically several representative handfuls . To measure soil texture, the sand, silt, and clay particles are sorted out by size and weight. The weights of each size are then expressed as a percentage of the sample weight.

In the field, soil texture can be estimated by extracting a handful of sod and squeezing the damp soil into three basic shapes; (1) cast, a lump formed by squeezing a sample in a clenched fist; (2) thread, a pencil shape formed by rolling soil between the palms; and (3) ribbon, a flatfish shape formed by squeezing a small sample between the thumb and index finger. The behavioral characteristics of the soil when molded into each of these shapes, if they can be formed at all, provides the basis for a general textural classification. The behavior of the soil in the hand test is determined by the amount of clay in the sample. Clay particles are highly cohesive, and when dampened , behave as a plastic. Therefore the higher the clay content in a sample, the more refined and durable the shapes into which it can be molded.

Another method of determining soil texture involves the use of devices called sediment sieves, screens built with a specified mesh size. When the soil is filtered through a group of sieves, each with a different mesh size, the particles become grouped in corresponding size categories. Each category can be weighed to make a textural determination. Although sieves work well for silt, sand, and larger particles, they are not appropriate for clay particles. Clay is far too small to sieve accurately; therefore, in soils with a high proportion of clay, the fine particles are measured on the basis of their settling velocity when suspended inwater. Sinceclays settle soslowly, they are easily segregated from sand and silt. The water can be drawn off and evaporated, leaving a residue of clay, which can be weighed.

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Thanksgiving is celebrated in the US on the fourth Thursday in November. For many Americans it is the most important holiday apart from Christmas. Schools, offices and most businesses close for Thanksgiving, and many people make the whole weekend a vacation.

Thanksgiving is associated with the time when Europeans first came to North America. In 1620 the ship the Mayflowers arrived, bringing about 150 people who today are usually called Pilgrims. They arrived at the beginning of a very hard winter and could not find enough to eat, so many of them died. But in the following summer Native Americans showed them what foods were safe to eat, so that they could save food for the next winter. They held a big celebration to thank God and the Native Americans for the fact that they had survived.

Today people celebrate Thanksgiving to remember these early days. The most important part of the celebration is a traditional dinner with foods that come from North America. The meal includes turkey, sweet potatoes (also called yams) and cranberries, which are made into a kind of sauce or jelly. The turkey is filled with stuffing or dressing, and many families have their own special recipe. Dessert is pumpkin made into a pie.

On Thanksgiving there are special television programs and sports events. In New York there is the Macy's Thanksgiving Day Parade, when a long line of people wearing fancy costumes march through the streets with large balloons in the shape of imaginary characters. Thanksgiving is considered the beginning of the Christmas period, and the next day many people go out to shop for Christmas presents.

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Arathersurprising geographical featureofAntarctica is thata huge freshwater lake, one of the world’s largest and deepest, lies hidden there under four kilometers of ice. Now known as Lake Vostok, this huge body of water is located under the ice block that comprises Antarctica. The lake is able to exist in its unfrozen state beneath this block of ice because its waters are warmed by geothermal heat from the earth’s core. The thick glacier above Lake Vostok actually insulates it from the frigid temperatures on the surface.

The lake was first discovered in the 1970s while a research team was conducting an aerial survey of the area. Radio waves from the survey equipment penetrated the ice and revealed a body of water of indeterminate size. It was not until much more recently that data collected by satellite made scientists aware of the tremendous size of the lake; the satellite-borne radar detected an extremely flat region where the ice remains level because it is floating on the water of the lake.

The discovery of such a huge freshwater lake trapped under Antarctica is of interest to the scientific community because of the potential that the lake contains ancient microbes that have survived for thousands of years, unaffected

by factors such as nuclear fallout and elevated ultraviolet light that have affected organisms in more exposed areas. The downside of the discovery, however, lies in the difficulty of conducting research on the lake in such a harsh climate and in the problems associated with obtaining uncontaminated samples from the lake without actually exposing the lake to contamination. Scientists are looking for possible ways to accomplish this.