河北专升本英语真题练习

　　B、club

　　C、climb

　　D、debt

　　试题答案：B

　　第5题

　　请从下列划线部分选出发音不同的一项

　　A、explained

　　B、hoped

　　C、changed

　　D、dreamed

　　试题答案：B

　　第7题

　　Diamond is the hardest material on Earth. Yet, gem cutters shape them. If diamonds are so hard, how do they cut them into shape? They use other diamonds of course!

　　Not all diamonds are the same. Some are harder than others. The hardness of a diamond depends on two things: its purity and its structure. Let's discuss purity first.

　　Diamonds come in many colors: grey, white, yellow, blue, pink, purple, orange and black.

　　Red diamonds are the rarest of all. This makes them worth a lot of money. Yet, pure diamonds are colorless. They are all carbon. They are transparent like glass.

　　Colors found in diamonds come from impurities. They have another element besides carbon. They have a carbon defect. For example, yellow diamonds have nitrogen in them. The nitrogen is yellow. Blue diamonds have boron trapped in their crystal structure. The boron is blue.

　　The purity or color of a diamond is important to its hardness. Pure diamonds are harder than impure diamonds. But purity isn't the only thing that matters. Structure also affects hardness.

　　The carbon atoms in a diamond are arranged in a crystal structure. Some crystal structures are perfect. A diamond with a perfect crystal structure is harder than one with flaws. So pure, colorless crystal diamonds are the hardest of all.

　　Good gemstones should be hard. This is what makes diamonds so great for jewelry. Some gems, such as azurite, are soft and easy to scratch. Azurite looks pretty, but it will scratch if worn daily. Diamonds do not scratch easily. Only a diamond can scratch another diamond.

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