Read the following text to answer questions number 4-6

    Gold is a precious metal. Gold is used as ornaments or as money. Gold is found in many places, but in a small supply. It is often found on the surface of the earth. Since gold is a heavy substance, it is sometimes found loose on bottom of rivers. The gold is found together with sand and rocks, and must be separated from them.

    It is simple to search for this type of gold. It is not usually necessary to drill for gold, but when a layer of gold is located deep below the surface of the earth, it is possible to drill a hole into the ground. Engineers have developed modern process for removing gold from rocks. Since gold is not very hard, it is sometimes melted and added to other substances for making rings, coins, and art objects. It will be priced forever because it is beautiful, rare, and useful.  

“It will be priced forever because….” The underlined word means ….