問題詳情

Questions 21-29Under the Earth's topsoil, at various levels, sometimes under a layer of rock, there aredeposits of clay. Look at cuts where highways have been built to see exposed clay beds; orlook at a construction site, where pockets of clay may be exposed. Rivers also reveal clayLine along their banks, and erosion on a hillside may make clay easily accessible.(5) What is clay made of? The Earth's surface is basically rock, and it is this rock thatgradually decomposes into clay. Rain, streams, alternating freezing and thawing, roots oftrees and plants forcing their way into cracks, earthquakes, volcanic action, and glaciers--allof these forces slowly break down the Earth's exposed rocky crust into smaller and smallerpieces that eventually become clay.(10) Rocks are composed of elements and compounds of elements. Feldspar, which is themost abundant mineral on the Earth's surface, is basically made up of the oxides silica andalumina combined with alkalies like potassium and some so-called impurities such as iron.Feldspar is an essential component of granite rocks, and as such it is the basis of clay.When it is wet, clay can be easily shaped to make a variety of useful objects, which can(15) then be fired to varying degrees of hardness and covered with impermeable decorativecoatings of glasslike material called glaze. Just as volcanic action, with its intense heat,fuses the elements in certain rocks into a glasslike rock called obsidian, so can we applyheat to earthen materials and change them into a hard, dense material. Different clays needdifferent heat levels to fuse, and some, the low-fire clays, never become nonporous and(20) watertight like highly fired stoneware. Each clay can stand only a certain amount of heatwithout losing its shape throughsagging or melting. Variations of clay composition and thetemperatures at which they are fired account for the differences in texture and appearancebetween a china teacup and an earthenware flowerpot.
21. The author's main point in paragraph 1 is that clay deposits
(A) conceal layers of rock
(B) can be found in various places
(C) are usually small
(D) must be removed from construction sites

參考答案

答案：B
難度：非常困難0
統計：A(0),B(0),C(0),D(0),E(0)

內容推薦

內容推薦