47. We take our smoothly paved highways and streets for granted as products of technology andengineering. They are made of concrete, a building material composed of cement, sand orgravel, and water.
(A) The huge concrete channel supplied drinking water to ancient Rome.
(B) The cement in concrete has a long history. The Egyptians burned chips from the GreatPyramid and used the resulting ash as mortar.
(C) The Romans made a cement out of volcanicash, which they turned into concrete by mixing it with water, lime, and rock; they used cementin the Great Aqueduct about 2000 years ago.
(D) Today, concrete mix containing sand is usedfor smooth applications; gravel mix is used where a stronger concrete is required.