【单选题】

Under theEarth’’s topsoil, at various levels, sometimes under a layer of rock, there am　　　 deposits of clay. Look at cuts where highways have been built to see exposed clay beds; or　　　 look at a construction site, where pockets of clay may be exposeD、Rivers also reveal clay along their banks, and erosion on a hillside may make clay easily accessible.(5)　 What is clay made oF TheEarth’’s surface is basically rock, and it is this rock that　　　 gradually decomposes into clay. Rain, streams, alternating freezing and thawing, roots of　　　 trees and plants forcing their way into cracks, earthquakes, volcanic action, and glaciers-all　　　 of these forces slowly break down theEarth’’s exposed rocky crust into smaller and smaller　　　 pieces that eventually become clay.(10)　 Rocks are composed of elements and compounds of elements. Feldspar, which is the　　　 most abundant mineral on theEarth’’s surface, is basically made up of the oxides silica and　　　 alumina 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 decorative　　　 coatings 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 apply　　　 heat to earthen materials and change them into a hard, dense material.Different clays need　　　 different 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 heat　　　 without losing its shape throughsagging or melting. Variations of clay composition and the　　　 temperatures at which they are fired account for the differences in texture and appearance　　　 between a china teacup and an earthenware flowerpot.The passage supports which of the following conclusions

A、Clay deposits are only found deep in theEarth.
B．If clay contains too much iron it will melt when fireD、
C．Only certain types of clay are appropriate for making china teacups.
D．If sufficient heat is applied, all clay will become nonporous.