2014托福考试阅读模拟试题训练（17）

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　　Questions 23-33

　　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 focusof examination in studies of soil texture. Texture is the term used to describe theline composite sizes of particles in a soil sample, typically several representative handfuls.(5) To measure soil texture, the sand, silt, and clay particles are sorted out by size andweight. 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 andsqueezing the damp soil into three basic shapes; (1) cast, a lump formed by squeezing　(10) 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　(15) 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 (20) 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(25) basis of their settling velocity when suspended in water .Since clays settle so slowly,　they are easily segregated from sand and silt. The water can be drawn off andevaporated, leaving a residue of clay, which can be weighed.

　　23. What does the passage mainly discuss?
　　(A) Characteristics of high quality soil
　　(B) Particles typically found in most soils
　　(C) How a high clay content affects the　texture of soil
　　(D) Ways to determine the texture of soil

　　24. The author mentions "several representative
　　(A) the range of soil samples
　　(B) the process by which soil is weighed
　　(C) the requirements for an adequate soil  sample
　　(D) how small soil particles are

　　25. The phrase "sorted out" in line 5 is closest in　meaning to
　　(A) mixed
　　(B) replaced
　　(C) carried
　　(D) separated

　　26. It can be inferred mat the names of the three　basic shapes mentioned in paragraph 2 reflect
　　(A) the way the soil is extracted
　　(B) the results of squeezing the soil
　　(C) the need to check more than one  handful
　　(D) the difficulty of forming different　shapes

　　27. The word "dampened" in line 15 is closest in　meaning to
　　(A) damaged
　　(B) stretched
　　(C) moistened
　　(D) examined