托福考试

【单选题】Questions 23-33The mineral particles found in soil range in size from microscopic clay particlesto 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. 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 sampleweight.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 a sample in a clenched fist; (2) thread, a pencil shape formed by rolling soil betweenthe palms; and (3) ribbon, a flatfish shape formed by squeezing a small sample betweenthe thumb and index finger. The behavioral characteristics of the soil when molded intoeach of these shapes, if they can be formed at all, provides the basis for a generaltextural 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 refinedand durable the shapes into which it can be molde
D、Another method of determining soil texture involves the use of devices calledsediment sieves, screens built with a specified mesh size. When the soil is filteredthrough a group of sieves, each with a different mesh size, the particles becomegrouped in corresponding size categories.Each category can be weighed to make atextural 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 thebasis 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 weighe
D、 The word "fine" in line 24 is closest inmeaning to
A、tiny
B．many
C．excellent

D、various

网考网参考答案：A
网考网解析：
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